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I'm using the ultraboard (1/2" thick) to make the screen. It has 2 sides and I want to paint one side gray so I can flip it around whenever I want. That forced me to come up with a frame that has a slidable edge so I don't have to unmount the entire frame everytime I switch out the screen.

Main challenge: ensure the corner is as tight as possible.

Here is the idea:

Using miter-saw set at 45 degree angle, I cut the 2" strip of plywood and mark the order of the pieces (to ensure perfect match). I then held them in place using C-clamps then put screw on them. It worked out nicely. The corner is very tight as you can see in the picture. I can slide the left edge in and out with a little force.