This is my entry into the 3D Printing Nerd's Spool Holder design challenge. It's designed specifically to fit Joel's filament racks, so I don't know how much good it'll do anyone else, but have at it ;)
The basic idea is that the hangar hooks over the front slat, and pushes up against the rear slat for support and stability. I didn't realize until after I'd designed it that Joel's slats aren't actually screwed into the rack, so it's possible it could pop the slat out of position if, say, he tries to use it on an empty rack. Given how much filament he's got, though, I doubt that'll be an issue ;)
I do worry about the strength of that top hook a little. I thickened it up quite a bit, but I may come back and do another design where the whole thing sits over the top of the rack. The contest doesn't say you can only have one entry per person, so we'll see.
This is a pretty big print (about 13in long), and I don't actually have a printer that can do it (hence no printed pics), but Joel has a GMAX, so he should be fine ;)