You should only smell gas if the gas liquid or fumes were to escape out of the fuel cell into the interior. The cell itself should not leak due to cracks or breaks in the container.
On a fuel cell there will be a few openings that could allow fumes or liquids to get into the interior but if properly done with the proper materials this shouldn't be a problem.
One spot will be the outlet where a fuel line attaches to. This runs to the fuel pump and then to the motor which if done with the right materials and tight connections shouldn't allow gas liquid or fumes out of it. If you don't tighten the worm clamps onto the rubber line or tighten the AN fittings they will leak.
Another spot is the filler opening where you add more fuel into the fuel cell. This should have a sealed and unvented style cap which this too should not allow liquid or fumes to get into the interior and last the cell should have a vent tube which would run off of the high point of the tank with the other end of the vent tube on the exterior of the vehicle and this is where liquid or fumes would exit. If the vent is vented into the interior then you will have a chance of fumes or liquid out of the cell to be in the interior of the vehicle. The vent tube should be the only thing that can allow vapors to come out of it and into the atmosphere. A properly designed and installed vent should allow air into a fuel cell and only allow vapors out. No liquids should come out of a vent.
I haven't gotten real far on my 73 hatchback but I do have a fuel cell for it and I looked at a hatchback Vega or Monza years ago and they had built a steel or aluminum wall and had it sealed on all three edges in the hatch opening and on the top they used a piece of weatherstrip to seal the topside up when the hatchlid was closed. Whether it was legal for racing or not I don't know and rules have changed but when the hatchlid was open you could get at the fuel cell, fuel pumps, and batteries easily. Since the area would be hard to seal up with metal against the long factory plastic side panels I would get an idea of where the wall would be and then trim off the plastic panel to where it would end up against the panel on the back seat area but in the more rearward section past the wall on the left and right of the fuel cell there would be no decorative plastic trim panels in there. With the plastic panels out of the way back there it would be easier to seal up.
I've seen some installs also with a fuel cell halfway through a trunk or hatch floor and a person could also design some type of 5 sided enclosure to go around the 4 walls and have a top on it all made out of steel or aluminum.