I usually get a rain of sunflower seeds when I raise my sail at the dock. (Bird feeder + innovative chipmunk needing sunflower seed storage). "Fighting" Mother Nature usually ends up frustrating -- it's more effective to work
with Mother Nature.
Bats aren't too fussy where they "hang out". I've found them in my woodpiles, eaves, under the house, and under my mailbox. They're also beneficial critters, and would probably prefer roosting in a bathouse that you can hang up near where your sails are stored. (Positioning it on a tree is important, though -- no sun during the daytime).
Bat houses can be pretty elaborate -- if you're handy with a saw. Here's plans for an economy model, like mine, which has concentrated my bat "problem" on a tree
away from the house.
http://www.batcon.org/bhra/economyhouse.html
'Course, you can always
buy one on the Internet or hardware store.
Or, this late in the season, you can leave off the cover and flake the sail on the boom with bungees.