On occasion I carry a 3mil clear plastic painters drop cloth (package unopened) just in case I got caught in an unexpected rainstorm. I think it was 10ft wide and 20 ft long. It saved me one time on the way home from the WSCB in 2007. Very compact, essentially a throwaway item, and very inexpensive, and waterproof. For long term storage (Winter) I also put it OVER the flannel cloth cover that I use in the garage (Shed to you Southerners) it keeps the dust from getting through. Never had an issue with the paint.
Like this:
http://cgi.ebay.com.au/Film-Gard-10-...3286.m63.l1177