I wanted to use heavier leather like veg tanned for the reinforcement, but I was just going with what I had on hand. I glued and sewed two thicknesses of the lightweight jacket leather together for the reinforcement and the back piece, but it's still not nearly as rigid as it seems it should be.

The 18 inch circle idea is interesting. Would you add a flap to it?
I don't have a big enough piece of leather to try that right now, but it could look really good with a great kilt. Must be really roomy, too.