I have found that I believe the 50mm or just another smaller rings work for me?! But I am kinda “high and tight” guy. I am thinking I want to have a custom metal cage made. But just don’t really seem to have measuring for ring down. also wondering if the oval rings are better? many one have a recommendation on custom cages?
I am also "High and Tight," and am very difficult to fit for a ball-trap chastity device. You may do better with a piercing and a tube-only device, which is what I finally did. Check my photos.
I personally like the Mature Metal line up, I would just go with their Jailbird model. You can also buy sizing rings from them before you put your final order in. You can order them in any size in 1/8'' intervals.... If you know mm well 2 inches is roughly 50mm, 1 3/4'' is roughly 45mm etc... I have a round ring from them but most people prefer the oval from MM.