What you are experiencing with the Castin’ Craft resin has been my experience as well. For what you want to do, the Resin Obsession super clear resin works great. It cures very hard and doesn’t get soft with body heat. I love using it for my thin band rings! You can see a sample here: https://shop.resinobsession.com/products/clear-silicone-statement-ring-mold