Missing Testicle Causes Device to Slide Off

My husband and I are total beginners to male chastity, and we need assistance resolving what is likely a rare issue. We began using our first piece of equipment this week (the CB6000s). The trouble is that because I lack my right testicle (it was surgically removed due to torsion), there is no ball on that side for the ring to catch, causing the ring — and eventually the entire apparatus — to slide forward and fall off. Right now, I am using the smallest spacer option, and one size down from the largest appears to be the minimum I can manage (for now). With such a tight gap, my left testicle (which was surgically anchored to the sacrum to stop another torsion) doesn’t move much and often feels compressed, puffy, and sore. A keying device wouldn’t seem to solve this, since the issue isn’t about my penis escaping. Neither piercing nor getting an artificial ball are choices for me right now. With all that in mind, is there anything my wife and I can do to keep this apparatus on? We welcome any thoughts or suggestions!