While there is a wide range of bezel sizes for turning coins into jewelry, it is difficult to find a 21 mm bezel. APMEX offers a 21.5 mm bezel that a 21 mm coin like the Rooster 20 Franc Gold coin could fit in with some wiggle room. For a snugger fit, you can put a thin “shim” of paper or plastic on the edge of the coin to help fill in whatever space is left to keep the coin in place.
If you’re flexible, you should check out APMEX’s selection of bezels to see if one could fit another coin you have.