I picked up a parva tank as soon as they went "live," and love it! The inner o-rings are pretty tight, but I'm not complaining one bit. I like that it's tight, doesn't slip off the carto even if people who I let try my PV hold it by the tank. I use a slap yo mama tool, and still get a little bit of o-ring slippage, but a little jiggle and wiggle and it goes back into place! However, sometimes my tank gets stuck on my Provari from refilling. I'd have to wait a few hours to a day or two for the o-rings to squeeze out the juice I used to lube it.
As far as suggestions to improve the inner o-ring issues, maybe (if they even make o-rings like this) an o-ring that is a little thicker, and has a "flat" outter edge to fit in the groove. And having the inner part of the o-ring round like it already is so that the it slides in as it does. But I don't even know if they make o-rings like that... Maybe a new project? LoL