I wanted a spool inside the printer cause i don't have much room around the printer for a outside spool holder so i made this one that fits where the factory cartridge went.
4/28/2015 update v2
I added a handle so you can pull out the holder easier and i add a filament dust filter i used http://www.thingiverse.com/thing:492067
filter and i integrated it into the spool holder
i printed at 50% infill and it is plenty strong enough. You will have to take the back off where the cartridges go and use a dremel or something to flush cut off the spins of plastic and the top lip to have the filament roll fit loosely as shown in the picture.