If you have any empty perscription bottles lying around, just rinse one out, drill some holes in the lid and pop the brush in there. If you don't have any, a pharmacist should usually be willing to give you an empty or sell it at cost (under a buck).
This idea isn't mine, it came from McNutt on SMF but I thought I'd share it with you guys. I know I'll be doing this when I go on vacation.
Another idea (tryphon @ SMF) was to drill a single hole in the lid and a few more on the bottom of the box to prevent bristles from sticking out.