I removed the package tray from my '81 Firebird......the front part (right behind the seat) was glued to the metal frame underneath. That is all...the rest just lays there....slides under the trim piece below the glass. Not sure if its the same on a '75 Nova but I wouldnt think its much different.
If the original package shelf cover is flat here's what I have done on customer's cars:
HOW TO MAKE YOUR OWN NEW REAR PACKAGE SHELF SO THAT THE SPEAKER GRILLS ARE NOT VISIBLE
First we need a template. You can either use the old package shelf or make a new pattern out of cardboard. Do not use poster board as this is too thin.
Take out the old package shelf and then lay that or the new template onto a piece of 1/4" Luan board or tempered masonite. Trace around the edges and then cut out the 1/4" board using a fine tooth jig saw blade on the marks you made. Make sure the template of pattern does not shift while you are tracing around it.
Next, put the new 1/4" thick package shelf back into the car and check it for fit. If it’s too big in spots, trim it down and then recheck your fit. If it’s too small then another piece of 1/4" board needs to be cut. You are wanting a fairly tight fit against the back glass, both the left and right sail panels and also up against the back of the rear seat.
The 1/4" thick board later will be covered with speaker cloth so it is important that the board fits as best it can. The edges by the package shelf normally go underneath a small amount under the side sail panels and into a groove by the back glass. If you do not spend the time to fit the board now it will not look any nicer once it is covered.
Now that it’s fitted and we have a good base to build off of, put the 1/4" board back on top of the package shelf and then from the trunk side mark the bottom side of the Luan board as to where the speakers an/or defroster holes are at.
Next, pull the new package shelf back out of the car once again and cut out these holes. Take some sandpaper and make all of the edges smooth and even.
Now we could just cover the shelf with speaker cloth but there is no support underneath of where the speaker holes are at. To correct this I’ve used 12" waffle grills (the grills like they use on subwoofers to protect them), and after flattening the edges out so it is completely flat I then place them on a large table and then lay the new package shelf on top of them on the table so I can mark the waffle grills for where I need to trim them down to. Since most grills are black, you may need to use a white marker or a color that will show up on the black surface.
After I see where the marks are on the grill I do not want to cut on that line but I want to cut it about 3/4" outside of that line. This will make the grill larger than the opening so it doesn’t fall through. I cut the grills using tin snips and after cutting them to size, I flatten the edges again if needed so the grill is nice and flat.
Next we need to take a router and on the lip of the speaker openings on the 1/4" board, a notch or step needs to be cut into the top side of the wood so that once the grill is glued into this recess it will be flush with the surrounding wood material.
Now that the lip has been routered out, check the fit of the grills and make sure they are flush or a little lower that the surrounding wood. Now, take some glue (I use Liquid Nails) and apply the glue into the groove of the package shelf and then push the waffle grill material into it. Add some more glue where needed and smooth it down the best you can and then place a medium weight on top of the grill to hold it in place while the glue dries. A person could use a faster setting glue like epoxy instead but it depends on how fast you want to make the new package shelf.
If you used liquid nail or a slower setting glue, let the shelf sit a few days with the weight still in place and on a flat surface.
Now that the glue has dried, take some sandpaper or a power sanding disc and smooth over the edge of the wood, glue and the grills. After you are done, there should be no grill material or edges and glue sticking above the wood and speaker areas.
We are now done doing the wood work and I would then put the new shelf back in the car and make sure it’s still fitting and lining up with everything.
To help keep the new shelf from warping and also too keep from seeing the wood through the speaker grill cloth, I coat the whole new shelf with a paint to seal it up.
After the paint dries, it’s time to cover it with the speaker cloth. Speaker cloth is nothing more than a double knit material however it will not have any kind of UV protection. To help keep the cloth looking new, purchase the grill cloth that already has the UV protection on it. Aamp of America sells this type of cloth in different colors and comes in a package that will cover a 36" by 66" area.
Next take a piece of speaker cloth and cut it down to where it is about 3" larger than the new package shelf. Now take a good spray glue and on the backside edges of the new package shelf, spray on the glue from the edge to about 2" in all the way around the 4 edges. Let the glue tack up and then in the mean time place the speaker cloth on a flat surface and then take and lay the new package shelf face down on top of and centered on the speaker cloth. Now what we need to do it put a little tension on the cloth so it is taut and not loose but not so tight as to bend the wood panel it is attached to. Begin by taking the cloth on one of the long edges and folding it over onto the backside and press it onto the spray glue. Do the same on the opposite edge but pull the cloth a little tight and then push it onto the spray glue. With a little less pulling pressure than what was done there, grab the cloth on the short edge and pull it up and push it down onto the spray glue. Now take the last edge and pull it tight and press in onto the spray glue.
You can now flip the package shelf over and see if you have enough tension on the material. It will be wrinkled in all the area’s except in the spots between the parts we have pushed into the glue already but those are the ones that will get worked out next.
To get these wrinkles out some pulling pressure must be applied to the cloth before it is wrapped around the edge. Continue doing this a little at a time until you get it stretched taut. With the spray glue if it’s not tight enough, pull the cloth off and put in a little more tension on the material. Eventually all of the wrinkles will be out and the cloth will have a little tension on it.
Now on the backside there will be extra cloth we need to trim off and secure more permanently to the wood. Take a new single edge razor blade and about 1.5" in from the edge cut through the cloth all the way around. This will leave a nice 1.5" border of cloth around the back edge of the package shelf.
I’m not sure of the longevity of some of the spray glues so what I do is take some of the left over Liquid Nail glue and put a bead of it all the way around the edge of the cloth where it ends onto the wood and where you cut off the excess. I then take it and smooth it over to where once it’s dry it locks the edge of cloth to the backside of the wood.
Since this step takes a day or so for the glue to dry, we need to keep the backside of the new shelf from rattling against the metal top of the package shelf in the car. What I do is put on a layer of speaker box carpet on the topside of the metal. To attach it to the metal I first cut a piece of carpet that fits and also leave a little extra flap to go behind the seat where it will be mounting later and I don’t worry at this point about the speaker holes. I then spray the whole topside of the deck with spray glue being careful not to get overspray on things that are close and then
also spray the backside of the carpet that will be going in there. After the glue tacks up I then position it on the deck and press it down until it is fully attached.
To cut out the speaker holes I use a razor blade to trim out the carpet that is covering those openings.
Now the car is ready for the new package shelf HOWEVER a piece of vinyl or carpet needs to be attached to the front edge of the new package shelf to fill the gap between it and the backside of the rear seat.
Looking at the pictures in the link below of how the new package shelf is made, you can see how this strip is first attached to the front lip of the new shelf and then folded back over on top of itself and then the rest of the strip is attached to the vertical part of the steel behind the rear seat. If you use the carpet no foam padding needs to be installed, but when vinyl is used a foam needs to be added as the vinyl is thinner than the carpet and will leave more of a gap once the back seat is reinstalled.
With the flap installed the new package shelf can be put into the car. Once in position, the front lip of the folded over flap needs to be attached with a spray glue to the vertical metal behind the rear seat.
The rear seat and speakers can now be installed to finish up the installation.