2nd Amendment Cyclist
Join Date: Oct 2010
Location: Cary, NC
Bikes: Schwinn 2010 World Street, Handsome Speedy w/ SRAM Apex
For a cheap, effective, cover, get a pack of shower caps - the clear disposable kind.
Two of them can be stuffed into just about any under-seat bag. Seriously, if you can't fit them in, you probably already can't zip the bag closed.
You take two - one for the Brooks, and one for your head (they go over helmets just fine). If one gets ruined, you let your head get wet before the Brooks.