I made my curved forms out of florist foam like this: https://www.walmart.com/ip/FloraCraft-FloraF-M-Foam-Disc-1-9-inch-x-9-8-inch-Green/593218875?from=/search
It’s easy to cut a large and small form out of one disc using a bread knife. I added a few layers of aluminum foil before the masking tape to keep it from melting and had no problems. After reading Leslie’s tip above, I would probably use a strip of silicone instead of the foil. A single silicone placemat would probably work great and be enough material.
It’s a good, cheap option for people who don’t have access to the tools to make them out of wood.