I bought my forst digital camera together with an underwater housing. The camera was an Olympus E-20 and the housing a prototype. Soon, I wasn't satisfied with the results (and quality of the housing) anymore.
The next step, having a housing built for the Olympus E-1 was a dead end. There were many reasons the project failed and I spent quite some dive without a camera. My next camera was an Olympus E-330 inside a UK-GERMANY housing. It was a big step ahead, but didn't fully meet my expectations.
With the help of UK-GERMANY I started to design a housing myself. In the final stage of the project I even designed printed circuit boards and wrote software for the electronic camera control. After nearly a year, in October 2008, I successfully finished the project with a test dive in Bali. Uwe Kiehl gave me the finished camera during a stop-over at Frankfurt Airport. The housing worked as expected. It even exceeded some of my expectations. You may have a look at some of the pictures I took with it in my gallery.
There's a simple answer (even if it isn't exactly targetting the question). "simple forget it".
It's cheaper to buy even another camera and a new housing than to build your own. There's no sensible explanation for building your own. Not even a good excuse. Its genuine madness. If you want to spend lots of your time, are willing to learn, able to accept failure, you might be the one to succeed in the end.
I spent a year of work on the project. Nearly every evening I sat at my desk, designing, calculating, drawing and throwing away dead end sketches. In addition, you have to calculate software license costs, electronic parts and material for prototyping. Without a commercial partner building and selling a small number of housings it would have been far too expensive for me.
You won't get rich with such a project. Not in a monetary sense. It's the satisfaction of a job well done. Having built something that lasts. Lessons learnt, that might come in handy sometime in the future.