A bungee cord will work as long as it's taut. I've used them, but I feel more secure with velcro. Electrical cord wrappers or tomato ties can be found nearly anywhere, and you can daisy-chain two or more if they're pre-cut to lengths that aren't long enough for your bike.
I'd be surprised if your bike rack allows your rear wheel to spin (but I don't know what kind you're using). Usually you only need to secure the front wheel.