This looks like a fine DIY project, especially the way you created the Cantles from savaged material. Recycle!.

I would suggest considering a wider gusset on you next attempt. I make my Rob Roys with a 3" gusset. If you are allowing 3/8" per seam times 2 seams, That leaves only a 1" effective gusset. I'd add at least 1/2", maybe a full inch. My major complaint about most sporans is lack of useful room.

cheers