This is a remix of a very simple spool holder that I liked, by Zweben.
I lowered and set back the actual holder, to be able to fit it in my Ikea Stuva enclosure, thus the "low rider remix".
I also added a filament guide, remixed from Ismael's holder and stiffened up a bit.
EDIT (2017.03.21): Although I actually never had a spool drop off the holder, smaller spools tend to wander off to the sides so I added an optional simple clamp to keep spools in place after loading them (print at 40% infill, single perimeter, 2-3 base and top layers).
20-30% (holder) / 40% (clamp)