I’m not an expert on bigger cages as hubby fits a CB-6000 well but it does get narrow before it gets wider. I’d have thought you’d be better off with something like this which is the same width (all of which is bigger than the plastic ones) the whole way along:
Here’s the link to it on Lovehoney – Fetish Fantasy Steel chastity cage
I guess if you find you don’t want the belt bit you can just remove it.
Because you’re a ‘grower’ though it’s really important you lube it well with something that doesnt just dry up, so make sure to get some good silicon lube.
Here’s a good selection of Silicon Lubes, I recommend the ID Millenium one and the Pjur is good too. If you haven’t tried silicon lube… well you really have to, it’s not just for cages, it’s AMAZING for play together. I’ll have to write up about it sometime! lol