I'm in the process of modifying my MakerGear M2 into a twin extruder setup as designed by Tim Edwards. Documented here:
One of the first things I needed was a second spool holder for the right side of the machine. The right spool holder that MakerGear has on GitHub is too large in diameter to fit spools with a 1-1/4" spindle hole such as Hatchbox. It's also too long at about 4-1/2". I found another spool holder that gets the diameter right but is even longer at 5"
This spool holder starts with the dimensions of the currently shipping factory left-side spool holder. It's only 3" long, and is the correct diameter to fit any manufacturer's spools. It has the standard (4) holes for mounting on the left side, but it also has two additional sets of holes so that it can also be mounted on the right side and at two different heights depending on your configuration. Works great.