The way I do it is cut a long string (roughly square) from a hide, give it a good soaking in distilled water so it's pliable and then
draw it through my rounder (a piece of hardwood with various size holes in it). After that, I roll it between 2 pieces of hardwood until
I'm satisfied that it has the shape I want.
If you don't want to go through that trouble, you can get various diameter climber's line from REI. It's not climbing rope, just the various
smaller stuff that's used to tie off gear. It doesn't stretch and makes an excellent core that's firm and will hold shape.