Just modified the Spool holder I had designed for my Two-Up, it hung off of my desk, to fit on my 3UP. This one comes in two parts, the holder itself and a filament loop that is optional. I like the loop for preventing tangles and it keeps the filament out over the extruder.
It snaps onto 20-20 extrusion.
2/29/16 Added a version of the holder that can attach to 20-20 Extrusion that is flat on one side.
12/25/16 Found on the 2/26 version that a 1KG spool will still bend and disfigure vertical post if it sits on it for a long period(weeks).
Added 10mm to post to stiffen it and subtracted 10mm from foot so it still takes up the same amount of space on the extrusion.
Added supports to the critical places so it can be printed in place at is with no supports otherwise, much less clean up.
Still works with current guide, my first print even appears to fit a bit tighter with the guide I already had on the old print.