Struggling with the Right Fit

Hi there, I just bought my very first HTv2 chastity cage. Since I’m new to this, I thought you simply “slid it in” and secured the lock. However, I have a selection of 4 different rings available. It’s possible to fit all four rings on me, but the largest one measures at 50mm & my measurements tend to run large. It’s odd that they still manage to work? Additionally, putting it in using the stocking technique is quite uncomfortable because it clamps down tightly from every angle, leaving a lot of skin exposed through the opening at the end. Lubrication helps improve things, but you don’t have much control over how it enters. Finally, when I attempt an erection, there’s a bit of bulging above the ring that feels firm? Apologies for what is likely a common question here, but I’d appreciate hearing whether this setup works or if my experience indicates it might not suit me! Cheers