I have my amp under the passenger seat. It's out of the way, and in no danger from kicking (it's robust in that direction). It's a cheap Sony one (NX440 or something - the box is now in the attic) as I don't need anything fancy to outperform the OEM system. There's nowhere better to place it in an estate car. Where it is is both out of the way and concealed from the exterior. There's no sacrifice of the flat load space (sleeping space, often as not).

When I next get to borrow a digicam, I'll post some photos on the Installation board.