Got tired of trying to get spinner caps to fit just right. So I made this one that fits itself perfectly every time. Feel free to take it and use it for your design. I've attached the design file. Modeled in Fusion 360.
EDIT 2017-05-05: Uploaded a 22mm diameter version as well as a 22mm diameter taller/thicker version. For the standard 20mm diameter tall version, see the remix by fishtacos: http://www.thingiverse.com/thing:2285212