PVC pipe works just fine. You can glue the caps with PVC solvent/glue then seal the seam with silicone if you want (but not really needed). A handy option would be a cap with screw plug at one end (with a little silicone grease). That way you can adjust the buoyancy a little before diving.
Adding anything inside, like Great Stuff, will actually reduce its buoyancy without adding any significant structural reinforcement. Besides, the 450 psi is internal pressure rating, external should be significantly higher. (Remember, circles and arches are extremely efficient at dealing with compressive forces.)
One other point, you'll want your buoyancy compensation to be above the center of gravity. At the top is fine, a little above the camera is fine, vertically along the sides is probabaly fine (as long as their exactly the same buoyancy), but you do not want it at the bottom. With the buoyancy at the bottom, it is a balancing act/battle trying to keep your camera tray in position.