I have my amps mounted to the back of the rear seats. and they dont leave massive holes or anything either.

Come to think of it, most of the cars ive owned, the amp's have been mounted to the back seats.

I refuse to put an amp underneath my passenger seat. Thats like overheat city, imho. I see alot of ventilation problems for big monoblock amps happening there. I guess if you've got a small 2 channel amp, it couldnt hurt.

I also like the idea of an amprack. or that carpeted plywood idea. good ideas.