I am by no means an expert but what about mounting the glass on the opposite side? Right now you have the plexi being held in by glue for the inward pressure that is going to be exerted as you go down in depth. I would think you would want it on the opposite side with the hard plastic stopping it from moving in further as it tries to compress with depth.
How do you take the pictures, I don't see any buttons.
How did you mount the handles/latches? It looks like you used screws but only put them part of the way into the PVC , is that strong enough?
I thought about building a camera case, may still it would be a fun project but for now I just purchased one for a trip I just went on.
*edit* good call on the buoyancy question, that is a lot of air inside the case and I imagine it will float like crazy.