Right, I used 10mm MDF, as I wanted to use the boot at times for other things (quick and easy to remove subwoofer box).
I've basically used some 2 x 1 battons around the edges and accross the center on the bottom (metal) of the boot. This is bolted down to the boot floor, but I have learnt recently that Gripfill also does an equally good job and saves drilling holes in the floor). On top of this I've screwed down some 6mm MDF which is the lower section of the floor.
To this I've then screwed my amps, crossovers, power distribution blocks, etc. Also on this base I've screwed down some 2 x 3 battons, around the edges (screws go right down to the bottom battons for maximum safety and support).
This now gives me a box in which all my equipment can be placed, on top of which I've cut out the top MDF floor (10mm) which provides enough support for heavy weight in the boot, including the subwoofer box.
For cooling, although I've never had an amp cut out due to overheating (stays quite cool under the floor as there are ventilation holes in each corner at bumper level) I've used 2 x 6mm computer fan's at each end of each amp, 2 pushing air and 2 pulling which gives me a nice airflow.
The fans were just from ebuyer (a while back, only about 50p each) and these are wired into a relay setup which is used to turn on my amps, crossover, boot illumination lights, etc. When the headunit is switched on, the REM then activates the relay to give power to the necessary equipment.
Most computer fans are 12vdc so should be fine. I've also fitted a fan speed controller, just to slow them down a little.
Electronics tend to run well at 'normal' temperatures, however valve amps particularly benefit from being warmed up for 15 minutes before they sound their best.
Yes, if run hard then car amps can run rather hot. Class AB amps tend to run hotter than Class D as they are less efficient and draw more current. However, most amps nowadays have a thermal cutout option, so if you push them too hard, you just need to wait for them to cool and they should switch back on again.
You also really dont need that much cooling. My old setup didnt use cooling (3 big amps) and that was fine 