Lots of good ideas on here to hold the filament spool, but none quite what I wanted. This design is optimized for print speed and material usage. I had about 10% material shrinkage (YMMV), so I made the hole size a bit bigger than nominal. The "boom" (extension) can be printed and different lengths that best fit your printing needs. I used CURA for slicing as I think it has better output than Slic3r, and with 15% infill, its way stronger than it needs to be. This version has a bit thinner walls than my first one recognizing it was overbuilt (still is a little). You could probably do 10% infill on the main part and be plenty strong. There's no tendency for the spool to fall off.Designed in Solidworks printed with eBay PLA, in-use as I type this ;) I had to estimate parts fit based on material shrinkage - your materials and parameters may result in slightly loose or tight fit (or right on).