tennis balls hanging from the ceiling seem very practical. Hardest part is probably getting the rope/string into the tennis ball. Second hardest part is hanging it from the ceiling.
I would say getting the string into/onto the tennis ball is the easy part. Hanging the ball from the ceiling with the vehicle parked in the correct place is much more difficult, unless you have freakishly long arms.
And just throwing this out here, but I have a double door, and two single doors on the same garage.