another trick--if a shower cap isn't available--get a large bag from Disney and cover the controls. The really large bags also allow enough maneuverability to "work" the tiller through the bag and keep the entire thing dry. Also, remember not to drive through puddles if you can help it.
(I also keep those big alligator type black clips in my Owner's Locker to keep the bag secure so it doesn't fly off. But they would be easy to pack in a suitcase too.)