The ID if the 4" acrylic tube is the same as the aluminum (or as close as it can be with cast acrylic). Because it is cast acylic, the ID sometimes goes in and out so we test fit the tubes. Our technicians are looking for a consistent 1-2 mm of o-ring mate all the way around. If there is a break in the seal, the tube is thrown out.
I think about the only way to really test that on PVC is to do a vacuum seal test and then if that works try lowering it down to your operational depth without electronics inside and see if it leaks.
We haven’t tested our flanges on PVC, so you are a bit into unknown territory.