How to put on chastity properly?

My boyfriend and I have decided to engage in an FLR, and I am trying to put on a plastic chastity cage on him to begin with. We're both new to this, so we have little idea. His penis is small when he's soft (he's Asian), so we decided to start with smaller cages. But the problem is, he can slide the upper part of the ring at an angle to free his dick. We have tried with multiple ring sizes and 3 different cages, but the problem persists. Wondering if we're doing it wrong or if it is a fit issue. Any advice would be gladly appreciated.