Simple plastic insert, no need for supports, high infill or more then 2-3 perimeters on a standard 0.4 nozzle.

Notes:

• was initially modeled for the Alegoo PETG Pro cardboard spool, but make sure to measure your own spool and the outer diameter of insert shaft in slicer. (even if this is the spool you have)
  if you want to adjust size:
  (new scale %) = (needed diameter)/(current diameter). and then check again
   
• Be aware that this adds about 2.5 mm to the spool thickness on each side (in the current scale).
  *the second pic below show the important diameter to match the spool.
   
• For Remixing: both STL and STEP are included.
  Also, the onshape document is called “Alegoo inserts” by EitamR, should allow to make a copy and edit.
  Link to the onshape doc:
  
  https://cad.onshape.com/documents/40d0d2edb7b0bada1d3a91d2/w/da930a85b3046eebf39ccb42/e/a969422898448be37ec6e32f?renderMode=0&uiState=67c2034c5e220874807393ab

Finlay, all feedback and suggestions are most welcome!