This is my printable replacement for the stock reel holder that handles reels with smaller hole diameters. I bought several batches of PLA which came on reels with ~37mm diameter openings. They don't fit on the stock holder, but they fit on this one quite nicely.The STLs I uploaded will fit reels up to ~6.5cm wide and down to a ~37mm diameter hole. You can easily modify the length to handle wider reels via the OpenSCAD file. Version 2 (V2 files) has a slightly wider gap between the tongues that fit onto the Ultimaker in order to address the issue encountered by Owen. I haven't tried printing the new version, so if you do please let me know how it turns out. If I get confirmation it fixes the issue I will delete the old version.1) Print two arms, one holder, and one end. I used 30% fill for the arms and 20% for the rest (0.15mm layers, 3 skins, and 2 perimeters), which turned out to be more than sufficient.2) Place arms in bracket on back of Ultimaker.3) Place holder on arms with larger radius section in notches nearest the back of the Ultimaker.4) Place reel on holder.5) Place end piece in notches at end of arms.