I bought one of these. It's inexpensive and might not be durable enough for regular use, but the few times I use it, it's fine.
Team Obsidian: Heavy Duty Waterproof Bike Cover/Storage 4 Seasons Protection | eBay
I'm not too worried about rain, per se. But we sometimes go on gravel roads with the bikes on the back of our camper and, rain or shine, they're just absolutely filthy when we get there. If we were doing regular trips with the good bikes on the back of the car, I'd probably bite the bullet and get a rigid, lockable cover. But on most occasions, they're either in the covered back of a pickup truck, in the Subarus, on the back for short hops, or locked on the roofs of the Subarus (less filthy!). So we don't need the cover very often.