Check the new thing I made, it's improved version of this and customizable, http://www.thingiverse.com/thing:1411131
Fits for prusa i3 aluminium frame.
No other parts required, just print and snap them on.
I tested them with 17h marathon print, worked just like I imagined they would. Even up to 190mm+ printing height. The spool actually can turn in the holder, so it will turn a bit towards to the extruder, when it goes from side to side. Especially handy when printing near the max height.
there's some options to tune in .scad -file, but the stl files should be ok too...
You may have to use some force when installing the holders, like tapping with a hammer to get the frame fully in the "hook".