Prusa i3 spool holder shaft (MK2 or MK3 + alternative spool holder)

Spool shaft which fits MK2 / MK2S spool holder and allows you to use a wide variety of spool sizes. Also fits into MK3 spool holder https://www.thingiverse.com/thing:2763091 for improved MK3 spool holding. (IMHO the stock MK3 spool holder isn't good. Doesn't fit several spool shapes I own, also it tends to bounce / wiggle when printer is printing fast.)