Difficulties with wearing a device? discussion

Hello all, I just bought a Chinese HTV3 Nub clone made of stainless steel that comes with two rings: the 40mm one is a bit tight (it causes pretty bad scrotal swelling at night, though it’s fine during the day), and the 45mm one is quite loose (my penis slides out easily). I really like this device and plan to wear it for a long time. I’m thinking of increasing the size of the 40mm ring just a little bit—maybe by about 2mm—to get a better fit. Has anyone tried doing something similar? What tools or techniques do you recommend?