As a keen tinkerer myself, I built my own. The most important feature is to get the camera as close to the bottom as possible without shooting down. I found the best way to do that is to build the tripod around the camera instead of under it.
I destroyed a cheap sony VCT-R640 tripod (just use the legs) so that the legs are adjustable in length, and then attach it however you can depending on the housing. I used the same principle (and Sony tripod) for a smaller version for my Navigator 900 housing too. For the bigger housings (like the one i use now), the ability to adjust the angle of the back leg is very important from a practical point of view.