Here's another solution. Build your flag case like this one.
flagcase2.jpg
Your angles to the base are 45 degrees. I cut a groove in the base for the glass using a router table with a 1/8" diameter bit. I set the fence so the groove is where I want it on the base to align with the grooves in the two top pieces and set stops so that the groove is the correct length without coming out past the edges of the two top pieces. I can lower the base down onto the moving router bit. I use feather boards to insure the base stays tight to the fence. Screws up through the base hold the top in place and allow glass replacement if ever needed.