Are you aware of
Aero Pack's retainers? Not sure if you aren't or if you already ruled them out because they are bigger than the slimlines?
I've used both, and definitely prefer Aero Pack's over the slimlines. Having the turning part on the outside makes it easier to work with (no tools required), and I think would be less prone to an issue I ran into that did my last slimline in, the rocket landed (under chute) on a rock, which dented the retainer enough that the inner ring was impossible to remove. Had to hacksaw the retainer off to free the aft closure that was trapped inside it, switched to internal retention (
another Aero Pack part) for that rocket after that. I don't think the same thing would have happened with Aero Pack's outer ring, just seems stronger but I could be wrong.
Aero Pack does make their base part in
a few different sizes to accommodate different MMT materials, though I often find I still need to sand the MMT a bit to get them to fit well.