I mounted my amps in the spare tire well (JL 300/4 and 500/1). They mount to a board that is actually bolted to the rim of my spare tire. I can unbolt and raise the amp board to pull out the spare tire if need be.

By using the board on top of the spare, the amps are mounted right side up, with about 1 inch of clearance between the top of the amps and the trunk subfloor.

To help with cooling, I've installed a vetilation system with 4 x 80 mm fans to circulate the air in the "amp chamber".

I've run this setup for a couple of months now and have spot checked after long periods of operation - the amp heatsinks are maybe 30 degrees when I touch them - very cool operation.

My JL dealer "certified" my installation, indicating they are confident that the installation orientation of the amps and fan cooling system are adequate for proper cooling. The things they looked for were the inch of free space above the amps, right side up amp orientation and adequate air movement in the chamber.

Orientation won't matter aslong as you can get the heat off the heatsink (with airflow)

the airspace inside your amp shouldn't change do to orientation. it will be warm inside no matter what

in the spring I'll do some testing with some temp probes to determine if mounting the amp upside down has any effect on internal temps

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Inside my 96 Nissan Sentra:
Pioneer DEH-P8600MP
Pioneer Premier GM-X924 Amp Powering 2x JBL P632 midbass, w/ CerwinVega Tweeters, on Pioneer passive xovers
Pioneer GM-X922 Amp Powering a Pioneer Premier TS-W1041DVC sealed
Awaiting spring: Pioneer CD-635, Clif Design QX6500C, a 0g run and adding a Sony LS-1065g

Just did another 5-hour-straight trip with the system volume cranked most of the way. I can't say much about my hearing, but the amps were cool as could be at the end of the trip. You can probably get away with any orientation you want as long as there is enough air flow to cool the important parts.

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M-A3
2006 Audi A3: Nav Plus RNS-E, Focal K2P 165 UVs, JL 10w6v2, JL 300/4 and 500/1, Cascade

Air flow is key.

I currently have my amps mounted to the front of my speaker box (about an 80 degree angle) directly behind my rear seat. When my seats are in the up-right position, the amps only have 0.5" of air to breathe between them and the seat. In the summer time, after 20 minutes of playing loud, my sub amp gets hot enough to go into thermal protection (and the back of the rear seat is hot, and the heatsink is very hot). But, when the seats are folded down, the amps get enough airflow that they only get warm and do not go into protection. (Also, note that it is only my sub amp which goes into protection. The sub amp is running at the minimum 2 ohm load. My front stage which is 4 ohm stereo doesn't even get warm -that amp can handle 2 ohm stereo. So impedance is also a factor for the heat that will be produced)

So if you are going to mount them in any position other than flat, right-side-up, make sure that you have sufficient airflow.

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Nostalgia just isn't what it used to be.

It depends on the amp,I wouldnt do it it....but just keep in mind heat rises and if all the heat in your amp is stuck in a 1/4" gap between the circut board and bottom plate it will have overheating issues=clipping or cutting out.