Welcome to the Forum! You're on the right track being cautious about drilling back there. My first sub install in college I ran a self tapping screw a half inch into my charcoal canister under the metal cover I drilled into in the trunk. Not as dangerous as hitting fuel, but a costly mistake for sure.

I believe the EVAP vapor canister on the gen 5 is in the driver rear wheel well area, I think I saw it under there when I swapped stabilizer bushings. The gas tank/fuel lines are under the rear seat, and extend back into the trunk a little. Your best bet is to pull back the carpet and feel around. If components are under the carpet, there should be removable covers that expose them. Look under the car as well.

 

Thanks!

 

Welcomed to the site fan057.

The tank is pretty obvious when you look under the car; I'd draw it up and possibly put a hole up into the trunk from under to make some reference points.

 

Remove trunk insulation panels. What it will do, it will reveal a lot of double walled metal panels for you. Ones, where you clearly see there is no components behind them. Choose one suitable for your purpose.
To be honest, not sure why you need to drill anything. I tossed floor mat into the trunk and zip tied sub to it. Floor mats have teeth on their backing, gripping to the trunk insulation. Never had sub move even a millimeter, while I could easily take it all out when spare had to be accessed.

 

If you are very worried, you can use heavy duty velcro on a large piece of wood with a swing door/hinges for the spare tire. Weight of wood plus velcro should make it difficult to move.

 

You would then drill into the wood instead of the metal below.