@Clint though it's hard to see the exact shape at the bell end, I believe there is a continuous taper from the mouthpiece diameter to the output port diameter. So the idea was to create a ring of tabs that bow slightly outward. This gives just enough flex to get it in without much force, but keep it snugly in presumably without putting much deformation pressure on the tube inside due to the taper of the tube.
Fundamentally it's just three stacked rings: an outer 8.5mm diameter ring to keep it from migrating up the tube, the "plug" itself at 6mm, and a smaller ring on top of that to support the tabs and reduce the risk that they snap from too much flex. The tabs bow out 0.25mm, which is plenty to provide a snug fit. I can easily push it in by hand, but need pliers to pull it out because it's too small to grab with my fingers.
One hopefully minor issue is that water can and does pool in between the tabs; going to have to keep an eye on that. I might try a version without the middle tier, though I suspect it will pass more air. Would like to refine a bit more but happy to share the STL.