Get a one ohm stable class d amp and you just cut your chassis count by one.
So I have toyed with this some. Its just not possible, for the simple reason an amp will cook itself mighty fast. We messed around with some cheap used amps in small enclosures with pc fans and tubes etc and about 20 minutes is the longest we ever had one last.
I have seen amp enclosures that were liquid cooled looked like a stupid amount of work though.
Although I have completely submerged several amps and they kept working.
On a truck I think the best place would be the bottom of a truck box with holes cut in the bottom to allow airflow.