Here is my home made in-wall rack. It was of the "Hey, lets cut open the drywall and see what we can fit in there!" type of projects. I ended up with two 26u racks, much better than the drywall that was there before! (-:
It's made from 25mm MDF with the center section made from two 25mm sheets laminated together. I had to remove a wall stud to fit it in so the rack had to be strong enough to support the remaining wall and ceiling. At the moment cooling is by convection only, there are vents at the bottom and at the top and cold air is drawn in from the HT and warm air is expelled out the top. This isn't a perfect situation, and since getting a new Integra DTR-8.8 AV receiver that runs VERY hot, I have had to re-think the convection idea and will be adding some active cooling solutions, with the hot air vented into the office at the back of the HT which is also air-conditioned.
Access to the back of the rack is via my office and when the back is on, it's totally light-tight. Excuse the dusty equipment in the pictures, I usually get all the equipment out every 12 months for cleaning... this must at the 11 1/2 month mark!