[cycleguy55]

You're going in to fiberglass most likely, so sheet metal screws or anything of that ilk are definitely out. Same thing for rivnuts or similar.

You'll need to use nuts and bolts (a.k.a. machine screws) - something that fastens on both sides. Fender washers are a good idea to distribute the load as well. If you go through the trunk wall, you should probably use screws as short as possible and put vinyl caps on whatever length of screw is sticking out beyond the nut - or cut it off flush with a Dremel tool.