This is my mounting system for my minimalistic spool ( http://www.thingiverse.com/thing:8331 ), but it should work with most other spools with a bearing center, like "Simple spool system for a Mendel (or other 3d printer)" by brokentoaster ( http://www.thingiverse.com/thing:3962 ) and its derivatives, and "Another spool" by CCphoto ( http://www.thingiverse.com/thing:8208 ). It mounts to the back of a Cupcake and has a filament guide to lift the filament up and feed it down into the extruder.

In the pictures, it's holding about a pound of plastic. The spool can hold a full 5lb roll, but I wouldn't want to put more than 2 or 3 pounds on this mounting, because of the torque on the Makerbot's body panels.

The STLs are for M6 rods, but the scad file is parametric, so you can use whatever size you want.