Big hub

The stock Flashforge 30mm spool hangers are fine for spools with larger diameters. However, the stock retaining nut is so small (see picture) that spools with larger hub openings can fall off of the hangers when moving the bot around. The big hubs (see other picture) have the same 30mm thread and are wide enough to restrain even the largest spools.
22 Oct 14: Added a smaller version of the big hub nut. Should require considerably less filament but it's still large enough to restrain spools with larger diameter openings.

March 2015: There are reports of a larger 50mm spool holder tube appearing on newer Flashforge printers. Obviously these won't fit the larger tubes, so measure first.