These are simple o-rings created inside PrusaSlicer. I made them slightly larger than the o-rings Lukas used. They are 12.5mm diameter inside and 19mm diameter on the outside (2mm thick).
Printed w/ 100% infill with a 0.6mm nozzle, they work pretty well.
UPDATE: Added a photo of the slicer stuff since the 3MF shows solid cylinders for some reason
The author remixed this model.