I tried many available designs from those with bearings that like to unwind the spool on their own. To 3 and four piece designs that required assembly with screws or snap together. I never was really satisfied with any of them because one would work x brands of spools ok but not other and so on.

I thought I would design a simple one piece spool holder that is printed in one shot. Spool holders don't need bearings, the spool slides smoothly on the printed plastic. Besides you want to have a little static friction or your spool will unwind and tangle. I designed this spool holder to fit all of my spools of filament from small spindles to large, from wide to narrow and should fit 98% of spools out there. Simple one handed loading allow use of the other hand to hang on to the loose end of the filament.

See my Spool Keeper accessory for this holder here: http://www.thingiverse.com/thing:2036770

See my Filament Guide for this spool holder here: http://www.thingiverse.com/thing:2038850

The spool is the same dimensions as my Wanaho Duplicator 6 Spool Holder: http://www.thingiverse.com/thing:2002418

Updates:
V3 - Changed reduced the accessory mounting holes to give 3mm screws more bite.