Have you thought about something like this?
http://www.leer.com/Truck-Caps
Ya I know they're not cheap, look used. You can black out the windows and lock it up. Aluminum ones are light enough to put on and take off by yourself if it's a small truck, may need an extra pair of hands for a full size.
Vinyl cab covers work if it's a light rain or a short trip but they will eventually leak through the stiching. The contractor bags you're already using are likely the most effective and cheapest solution. What are your aesthetics worth to you?
FWIW, I take a backpack full of heavy duty big garbage bags and a few plastic tarps to every outdoor gig, just in case.