Amp is behind a lip on the box totally invisible
Sub faces seats. Simple & clean. The footprint for the amp is about 10x8 so you will need at least a 10" depth on the enclosure to hide the amp. You need at least that much area for any decent volume anyways.

I wouldnt try to slope the box on the inside - facing the woofer towards the seat is bad enough, putting the woofer parallel to the rear seat will sound even worse.