UPDATE 15/8/2014 Added a V2 designed for a wider range of spools, this one will support spools with a 30mm or larger hole and up to 85mm wide.

I've already had to rip my Makerbot apart and voided the warranty, so modelled an external spool holder to use cheap PLA. Works awesome, no mods needed and prints pretty fast. Enjoy with your favourite aftermarket filament!
Designed for Bilby CNC PLA so the curve might not fit rolls much smaller than 52mm. Can CAD a smaller version if requested.

Print on its side so the slot is free from support. You will need support enabled to print this part. Seats securely on the metal backing of the Replicator, pull one of the filament tube ends free and feed your filament into it. I've found an additional holder for the filament tube isn't needed.