customizable bearing cap with finger pit

A parametric ball bearing finger cap. This is based on [bda](https://www.thingiverse.com/bda/about)’s “[Customizable comfy spinner caps. Cap for any bearing.](https://www.thingiverse.com/thing:2319653)”, but you have to put in some measures as length (in millimetre). The important ones:

* Shaft diameter: size of the inner hole of the bearing
* Bearing height: distance between the two flat surfaces of the bearing. The shaft of the cap will be 45 % of this and two caps will fit into the bearin