Mature Metal discussion

I get it, there are countless threads about sizing:) The silver lining is that many folks are working on getting their measurements correct! I am shopping for a MM - however my particular issue is that I often manage to slip out from the rear of most captive ball cages I’ve tested. I own the Jailbird v2, along with the Holy Trainer mini (and pico) - and when they loosen up and warm up, they simply … fall off. My penis exits through the back, and if I miss the window, it just slides out. My guess is that for sizing an MM, choosing a smaller ring will be key. The sizing kit is heading my way. Is it worth picking up two sets of rings? What should I look at regarding tube size / diameter, since I am usually small when relaxed, but clearly need some wiggle room if the ring feels snug (I figure)