This is just a simple 1" / 25.4mm high res sphere used as a caster for the XRP.
I'm not acting like this was a lot of work, but with no model provided on printables I thought I share mine. It can also be easily achieved in every slicer, but usually that model would be quite low res.
If you wanna see a bit more complicated models check out my other models like the boat mod, the track system, the off-road mod or the big boy.
It's intended to be a hard filament. Soft filament might cause too much friction in the mount. I would recommend using some organic support. Adding manual support on the model did not produce better results for me.
The author marked this model as their own original creation.