Don't remember from where this plastic "tube" come, but he fit over the pin that are in this place in gimbal and allow the 1/8 screw fit inside. Was fit only for test, after the pin place was bored and the screw bolted in the wall.
I was wondering why you did not drill the wall :P
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If you have a powered hand drill can reduce the pot axis tip with sand paper for metal, the pot axis is brass, relatively soft (will take some time) for fit the Bic tube. Bic tube easy the things due the ~parallel sides.
Thanks for letting me know that it is brass Power drill and a file here I come Just had a quick look at bic and pot shaft, there is not that much material to remove aprox 0.4mm.
Other option is to use acrylic tube OD 10mm ID 6mm. With 2mm wall thikness there is enough material to file a grove for magnets. If done properly hall sensor (4mm wide) would be sitting between 2 magnets with 1mm room on each side.
Bearings fited into gimbal that's lovely but without proper tools like stand press drill with vice I am not going to even think of doing it.
Power drill and a file here I come 
