Easiest thing to do is get a 16" square cut and then have it cut on the diagonal --- for the glass, source UV blocking glass --- any decent framing shop should be able to do this.

That's a good suggestion on the UV blocking glass

Be sure to ask for archival museum glass to best protect the flag.  A frame shop near you should be able to source it for you and cut it to your needs.  As for the flag case dimensions, the properly folded flag should fit closely, almost snugly, as though it was being held lovingly gently by the widow without allowing it to slump.   

I’ve done a full scale mockup and will send it to the widow. I want to honor this hero’s memory with the best work I can do.