Years back I started to make an oak and leather case for my Prussian Sauer. Yep, you guessed it, I sold the gun and never finished the case.

I can't tell of a source for 12Ga brass (approx. 1/8 inch). I bent and filed the brass corners to shape and had a local welding shop braze the corners closed. After the final finishing with 220 Grit wet and dry, the brazing is undetectable. It's really not as difficult as it sounds.