Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4598196.
Update: 2020-11-28: I noticed some wear on the Teflon tubing so I tried some 4mm polished stainless steel rods and the friction is noticeably bigger. However, it works ok for relatively full spools, but lighter spools do not work well.
Heavily inspired by the spool holders on Voron printers. Never seen PTFE being used for spool holding and I would like to give it a try. Designed to fit 4mm PTFE tubes and the stock I3 Mega S spool holder. My main concern is that the PTFE wouldn't be very wear-resistant, so I would like to try other materials such as polished stainless steel rods in the future. But if the Voron designers chose PTFE there must be a good reason.
Just print both halves (they are slightly different to avoid interference with the holder) and slide them onto the holder. It is a tight fit so don't be afraid to play with scaling and horizontal expansion.
Print with the flat side down, shouldn't need support. Material doesn't matter, I used 0 infill and 3 shells.