There are listings on eBay, Amazon, http://classiccardashes.com, etc. for 77-79 Nova 2-dr package trays. They're all molded from black ABS plastic (same as what Classic Industries is offering) so they have to be painted to match your car's interior color.
My '77 also has Firethorn Red interior and I had to freshen up some faded interior parts as well as re-color some pieces from a few different donor cars. I used SEM Color Coat #15113 Firethorn Red and it's a very good match to the original. SEM also offers a few different adhesion promoters for various materials (vinyl, polypropylene, ABS, etc.) And even with a new part, you'll want to give it a thorough cleaning to remove any mold release residue or other contaminants.