In my spare time I do some leather working. I used to do other textile crafting, but I like the more mechanical nature of how you fit and plan leather goods to achieve curves, seams, etc. I put together plans for a leather D20 shaped dice bag with a drawstring closure that is pretty easy to pull together from a single piece of leather.
The plans are available here and should be pretty straightforward (If you have any questions, let me know). The option to add stiffener triangles to the pattern can help to better define the triangle shape of the bag, or you can score the leather to encourage it to fall into more straight lines. I did both, as you can see in this photo of the bag mid-assembly.
It helps to punch the stitching with a hole punch tool as seen above. I used a mix of 3 and 4 hole punches to get straight lines, with angles that made stitching easier. Use a traditional saddle stitch to join edges, and a hole punch to pop out the lacing holes. I used a piece of scrap leather cord as a drawstring and the bag holds about a dozen mixed dice comfortably.
It also makes a satisfying rattling sound when you shake it.