I've designed a very simple filament spool holder, based on Z-top mount part (PI3A-M08) of the Geeetech Prusa i3 Pro B. It's actually very simple :
I cloned the part PI3A-M08 with Blender, making it a little taller, and screw the printed part on top of the original, using two M3*30 screws to secure it. Then I put a 15cm threaded rod (6, 7 or 8 mm), a spacer and nuts to hold the spool.
And voilà !
At first, I was afraid the printer would lose some balance, or the threaded rod to bend from the spool weight, but no, it seems to be working just fine
STL file in attachment.
Have a nice day !