Inspired by CCphoto's "Another spool" ( http://www.thingiverse.com/thing:8208 ), this spool is parametric, minimalistic, and designed to make changing filaments easy. To add a roll, hook the arm pieces around it, slide the rods into the slots in the arms, and tighten the nuts to clamp it. At the default size, it can hold a 5 lb roll as pictured, though it's probably better to use it with half the roll. On a Mendel, you could print one large enough for a 20 lb roll! It includes variables for the arm size and number of the arms (designed to work with 3 or 4, but 5 and 6 should also work) as well as the diameter of the bearings and of the rods.
I have included STLs for a version for m8 rod with 608 bearings and for the one I made, with m6 rod and some R188 bearings that I had lying around, both with 3 arms that will fit on a Cupcake's platform.
Print 1 hub and the appropriate number of arms. Attach with threaded rods 9-12" long. I'm using it with my minimalistic spool mount ( http://www.thingiverse.com/thing:8472 ), but most mounting systems designed for other spools should work.