Base ring sizing question

I am looking to get a kink3d cobra, but have a question on base ring sizing.

I have a knock metal cobra from Ternance with a 45 mm circle ring. It seems to pull forward and then skin at the top of my sack folds as it does. I don’t know if it’s the weight, or too big. But this is the first cage that I (initially) didn’t have any balls slip through (8 weeks, then I started getting some slips, but very sporadicly)

So now I’m looking at the original, and having trouble between buying a 40mm, or a 40mm curved, or a 45mm

Anybody moved from the circle to these rings have any guidance?

Thanks!