For anyone who might find this useful, I bought lengths of PVC pipe from the hardware store to use as forms. I have several lengths of 2’ length pipe which correlate to my wrist, my bicep/partner’s wrist, his bicep, and finally, my dog’s neck, which is a whopping 20” around. For that I had to buy a 2’ length of ABS pipe, 6” in diameter. It made a fantastic mold for me to make a collar. I taped the lining to it, and glued on the outer leather. It’s beautifully smooth inside, not a wrinkle to be found.
So try the hardware store or even a plumbing or gardening supply store (I think the big daddy pipe was for a pond or pool), and bring a tape measure with you.
If you can’t find the exact diameter you need, you can go down by an inch or two. Leather stretches a little easier than it compresses.