I scaled the original model's radius to 95%, added a little slope on the grip and made the part that wraps around the bottom of the stick a little wider.

Using flexible filament, it fits snugly on the bare thumbstick (without the rubber cap).

Printed top-side up with supports on the inside as Flexfill bridges poorly.

MK3

Prusa i3

No

Yes

0.07

100%

Printed at 0.07 layer height, probably more is fine as well.