DOH! I had other things on my mind and missed the compartment aspect of your question. I traced and cut out a piece of 4 mm okhume plywood from scraps using the opening of the homemade compartment. The pieces were S&G'ed, glassed on the outside (more scraps) and the inner joints filled with thickened epoxy. Everything was then given a couple of coats epoxy. I also fired up the old table saw and sliced up pieces or pine to produce 1/2" x 1/2" pieces to use as a frame that I will set on the inside of the compartment opening. The lid, when assembled will use some kind of knob with a metal latch that will grab the inside of the frame to hold the lid in place. The knob will pivot the latch which will grab the pine frame from the inside of the compartment. The lid will seal via foam gasket which I will cut from 1/2 closed cell foam (more scraps) cemented into place. I wanted a simple knob assembly such that I could quickly remove the lid to get a camera for quick shots. I will line the compartment with 1/2" closed cell to reduce impact damage to the electronic gear I keep in it. You never know when you'll come upon an unexpected rapid or something.
It's good to hear you are getting that trip together. You have to be sure to post trip reports and get some good pics. I know that area is totally awesome so they should be good. Time for this old techie to get on with his ambient air testing. Ah, life in the lab, filled with air not so sweet as the wind in the hair while paddling on a river or a lake. Six months to spring thaw.

Robert N Pruden