This is a more solid replacement for the Prusa MMU spool holder plugs. 

Needs a M3 cap head screw. The longer the screw the more firmly the plug grips the inside of the tube, up to the max length of 20mm. I find 14mm is more than sufficient 

I replaced my rods with OD=8mm ID=6mm Stainless steal tube and used these plugs 

Printed in PETG