Didn't see why mounting the amp under the passenger seat wasn't an option, besides some additional wiring. If not there, it appears that you need to consider using the back passenger side instead of the drivers side back wall. Passenger side has room for sub and an amp board.
-spill anything? there's a good chance it got on the amp
-the underside of the seat acts as a nice big insulator. Newer subs are better with heat, but it still isn't good sandwiching it between carpet and a big foam pad
-the dirtiest part of your interior is generally the floor
-people do a good job of kicking things.
This truck has limited space, and adding stereo equipment is a lesson in letting go. Your amp should fit fine on the back wall. I have a 4 channel and a mono block, the 4 channel is 18.5" L x 7.6" W x 2.4" H, the mono block is only slightly smaller. I fit that plus a 10" sub and I pretty much take up the entire space. it's just how it goes.
The other option is to custom fiberglass the under seat pockets and cut fans into the sheet metal for active cooling, but it aint cheap and you need small amps.
If you look at my sub box, you can see how I was stupid and thought I would save space. The box is made by Mr. Marv, so it's got proper dimensions, but I ate up a lot of room making the box short and squat.