You said you didn't want to drill holes because you felt the metal was too thin. You aren't opposed to drilling holes if they will hold are you?
Toggle bolts
The problem with toggle bolts is if you want to remove the screw, the toggle part stays behind and then you won't be able to get the screw back in. Here's a handy trick my dad taught me. Drill your holes and when you insert the toggle, squirt a bunch of RTV (silicone adhesive) on the toggle. When you tighten it up the silicone will adhere to the toggle and under side of the seat. You can remove the screws and when you want to replace it, the toggles will still be there waiting for you. You could also shoot some Great Stuff expanding foam in the holes just before you insert the toggles as it cures faster than RTV.
Just my 2 cents worth