I got annoyed at all the spool holders not being stackable so I made my own.

This is a 2x2 unit gridfinity bin 10 units high. 

For compatibility check that your spool has these dimensions:

65-66mm outer diameter (mine is 65.75mm)

20-21mm inner diameter (mine is 20.5mm)

60-63mm width (mine is 62.5mm)

Print out the body and two centers to hold the spool in place

I had to cut into the base to get the height to 10 units (I like the nice round number) but it doesn't interfere with the grids I've been using (standard and gridfinity refined)