Not fun dilemma. I don’t have any great ideas for a permanent fixture. It seems to me logically that if you would put any expanding foam or similar in there it would not prevent the shim from entering the leg as it would be soft and the forces are probably quite strong on the blocking nub. So if it started moving it would probably just wiggle itself into the foam/frame.
Wonder if the reason that 6mm ended up being the final size was that it is what (likely) makes the hub fit in a Mad4One frame. Or just because it’s simply strong enough to have a 6mm nub.
Hopefully I can soon confirm if the M4O frame fits the hub without drilling.
(and update the wiki with info 📜 [Wiki] Schlumpf 2022 Hubs - Frame Compatibility ⚙️ - it’s currently a bit outdated as it assumes the nub is 8mm)