I have the original filament holder mounted at the top of the frame using a couple of T-nuts and the original thumbscrews. I looked at loads of fancy spool holders on Thingiverse but no one seemed to have figured out the dead simple solution!
Simple, saves desk space, and it stops the control box falling over when you have a 1kg spool hanging on it. I use a short piece of ptfe tube just before the extruder to act as a filament guide too. If I was going to print something full height then I'd either move the spool or at least put the frame braces back to stop the top of the frame wobbling with an extra 1kg of weight up there, but since I rarely do print anything tall it doesn't matter.