A spool mount that sits on the top of an aluminium plate Prusa i3
Print two plate mounts and two spool mounts. Cut 2 identical length M6 threaded rods and fit to each, with nuts and bolts, a plate mount and a spool mount. Cut a length of M8 threaded rod to carry your filament spool (with spool adapters - http://www.thingiverse.com/thing:60723 ) and press fit it into the spool mounts.
This does add some weight to the aluminium plate and does make the frame bend backwards. This may affect prints but I am still testing that.
Check back to www.sub-design.co.uk/i3build/ to see how that pans out.
Ooops! Missed an STL - had two copies of the scad file for the spool mount. Added STL 29th March 2013.