First, I replaced the shipped spool holder on my Prusa I3 with this one:
Spool Holder for Prusa i3 MK2 - Hex Design
To prevent pulling the filament sideways off the spool, I needed a filament guide. I made the Filament Guide ClipOn by Area51 first, but after some usage, I improved a few things:
The loop was originally with a parallel gap between both halves, meant to close when put into the holder. But that cannot work this way because it then closes only at one end, the loop side remains open. I changed the gap to a "v", so now it compresses correctly and closes the gap when put into the holder. Also i extended it a bit for a better fit for the spool holder.
Next, I redesigned the ClipOn a little bit higher and more to the front, now a collision with the X-Axis is less likely when operating at high Z values, the hole for the filament storage is also a little bit bigger.
If you use very narrow spools, I made a variant without the filament storage.