Here's an ugly view of that old winterbeater Pathfinder that I had that I mounted an amp rack (and passive Xover network) in the roof of:
View attachment 14607
That amp was being pushed hard too, powering 2 subs, 4 midbass, 6 widebands, and eventually 2 tweeters as well, plus it's pushing everything through lots of passive Xover components.
This amp was running at 2 ohms for the subs and 2 ohms for the midbass - I don't recall what impedance the widebands were but there were three so I suspect about 2.6 ohms there as well - and it never broke a sweat.
The amp internals are still using the heatsink to draw off heat - and all you are doing is making it a tiny bit less efficient, by mounting it up where "heat rises" to. It's really not much less efficient than mounting it flat on the floor. The most efficient way is to mount it on a wall, because the rising heat will actually create a slight airflow - but again slight is the word.
It's a much bigger deal if you enclose it in a small space, or bury it with your gym bag, or mount it to your sub box and then shove that back into your rear seatback, etc.
No manufacturer ever asks "Where was the amp mounted?" in a warranty return.
