Staedtler (the folks who distribute Fimo polymer clay here in the US) had this great make n’ take at CHA using ring forms they are considering importing to the US. They have adjustable bands, contain bezels on top, and are perfect for filling with your favorite polymer clay. Hopefully they will be available for clayers sometime soon… see below for a sample of a square version. And here is a link to the ones they are currently selling in Germany, I believe. This is a lot easier than making your own ring forms!

Polymer Clay Ring