Actually, I'd say make the sporran square. Out of leather for the body. Perfectly square. Good solid sturdy leather.

Then affix a jigsaw cut of metal plates to it somehow. Not one solid sqaure plate for the front of the sporran, but a square cut in to 5 or six jigsaw pieces. Over black leather. It'd look like shattered metal bits if you did it right. The lid of the sporran should also be squared off with the jigsaw metal bits affixed somehow.