Mirko, the Flight stick model has 10 mm shaft, while the normal with ball or bat top has 9 mm. So my guess is that you ordered one with 9 mm.
I do not have the joystick yet. It's in my shopping basked, waiting to be checked out. Now I guess I'll wait for the result of the R&D process.
Monkey Ball is pretty decently emulated in Flycast. If you have the chance, I would be happy to know how you would compare an emulated setup + UltraStick to the real Naomi game with the original controller. I just played it once 15 years ago. Actually, if you find the UltraStik being too far off from the original, I might need to change my mind and complete my order with a Gundam stick. I do have a Naomi, but I actually prefer the simplicity of emulation as long it is done good.
If the diameter of the shaft is too small, perhaps you can thicken it using tape of some sort?
I will use a rubber gasket, but I'll order one together with the repro panel things from arcadeartshop.com. Though, if you happen to have a spare sealing ring, I'd be happy to buy it. Otherwise I'll 3d print one myself (though I doubt I'll make it as shiny as your banana, but the ring is not exposed in the same way).
I see you ordered the OG banana
Yes, given that OG means Gundam (No holes) c/w M5 screws