Just to kick these around for feedback, I have two approaches that I want to play with.
The first approach is straight forward, and one that I am relatively confident that I can do. This would involve replacing the thin black ring with a thicker one. One set of screws would hold the new black ring in place over the existing lense, just as the thinner ring does now. Once installed, this ring would stay in place. A clear polycarbonate lense (flat plate, actually) would attach with a second set of screws. This would provide an easily replaceable lense if it becomes scratched, but the 6 or 8 small screws would be a bit of a PITA if you wanted to take it on and off frequently when using the housing in or out of the water.
The second, and I think more elegant approach, is one that I don't yet know if I can do. It starts off the same, by replacing the existing thin black ring with a thicker one. Only in this case, the outside of the new black ring is threaded. A threaded cap containing the new lense and an o-ring seal would screw on to the OD of the new ring, just like the front of a waterproof flashlight. I like this, because it would be easy to remove the lense, without tools, any time you wanted to use the housing out of the water. And the "dryness" of the design has been proven in countless flashlights. It just remains to be seen if I can do the threads, and whether the whole assembly would stick out far enough to intrude into the picture frame.
Roll 'em around in your head, and look for flaws in either approach before I go forward.