Alpine system, non-e-torque here.
My powered compact Pioneer sub is mounted to a sheet of laminated mdf and then mounted behind the seat. There is a decent amount of room behind the seat, so a slim enclosure will fit back there. When the back wall carpet cover is back in place, there is some dead space still. If you remove that rear cover, then you will have plenty of room to work with. I went this route for the same reason…I have a WeatherTech under seat bin, did not want to lose under seat storage.
Only issue I can foresee is not having a total flat surface all the way across. There are tabs sticking out of the floor, thinking it’s probably for the reclining seat frames. Can’t remember how far they went back, but that might be an issue. That’s why my sub is on the wall.
Good luck