I generally use a white plastic bag from Walmart or Target. If you hang it high enough and far enough away from the trunk, it really doesn't matter.
Tried a RatSack; tried an Ursack; even tried an MSR Stowaway pot that latches down to form a waterproof seal. All are good, but are bulky, expensive, and heavy. Cheap plastic bags perform their function and are FREE and very light. Have to tie it up in a knot if it rains, but have never had my new bag invaded. So I get a little extra use before I recycle.