I built a clear case a few months ago, and I ended up not using it anyway. It was though to take the drives in and out, I had some problems with the plexi sagging in certain spots, and it took FOREVER to build. Then again I screwed mine together, I didn't glue it, but even if I had glued it, it would have still taken a long time. Also, all that plexi can get expensive, and after all the necessary supplies are bought, its going to cost over $100. And it you make a mistake, like slipping w/ a dremel for example, its very noticeable.
If you're really intent on building it though, I would recommend using 1/4" plexi, that should help avoid sagging. And you should glue it together. But, most importantly, make sure that you have a good blueprint to go by, and know exaclty how you're going to do things like put in your agp, and pci cards.
Anyway, I hope I haven't discouraged you too much, good luck, and drop me a line if you need any help