Somebody might have already built a box like this, but I thought I would through a couple pics out if anybody was interested.

http://www.geocities.com/millingtonbrian/photopage.html

It's just a flat bottom box leaving a big space under it. I've got two amps one bolted to the bottom of the box and the other is bolted to the floor directly underneath it with the fins going left to right. The two ports towards the rear of the car have fans at the bottom, one pushing the other pulling. I've got a cap and fuse block down there too. I'm working on making one fan slide out of the way to fit my hand down through the port to the fuse block directly bellow. Sorry no pics of under the box yet.

 

The amp bolted to the bottom isn't upside down is it? Amp's aren't designed to be mounted upside down. Mounting it upside down will cause it to retain heat. While this may not make it hot enough to go into thermal protection mode, it will help to shorten the life span of the amp's output transistors. Even with the fans the insides of the amp may be getting hotter than they would if the amp were mounted rightside up.

My $.02 FWIW

 

The pictures on that site don't work.

 

This is true, but can be compensated for with enough airflow over the Heat-Sink of the Amp. Controlled Forced Convection is a LOT better than uncontrolled free convection.

 

Its a free web page so only one or two people can look at it within a hour.

I did have a problem with amp thats upside down over heating at first. I put a little better fan in and I'm going to mount both amps on a thick piece of metal to help disapate heat, or I might put spacers between the amp and the box to allow air to flow across both sides of the amp. Thanks for the info I didn't know there not supose to be mounted upside down.