Unless the water repellent layer is a deal breaker.... a 30"x60" polyester beach towel is $10 at WalMart. Cut a slit for the head rest and you're done.

Add mini bungies if you want to take the whole thing to the next level for a snug fit around the seat.

Or if you're crafty, buy a couple of yards of the water-repellent backed material at a fabric store. Whether you hem it or not is a matter of taste and requirement.

You can do either of these options three or four times before you spend what's asked for in the examples provided.

$0.02