Johanna Johanson
I have made several of these. I don't reinforce the corners because the case, once the flag is installed, is not likely to be opened again and because the plywood back is screwed to the sides and bottom.

When I make one for somebody, I leave the back off (but pre-drilled for the screws) and send it with screws and a screwdriver of the correct size. Most people do not have a screwdriver for screws that small.

The last one I made, I added an additional display case attached to the bottom to hold medals, dog tags, etc.