I'm wanting to display temporary signs on the side, something like this:
http://www.masterpiecebanner.com/frame/
But, since the side of the bus isn't flat, I think I need to put the banner on a rigid backing, like 3/8 plywood, that would be held by some kind of metal frame attached to the side of the bus. I want to be able to slide the "billboard" out of the metal frame, when I don't want to drive around with a sign on the side. I was thinking I'd use some kind of aluminum channel on the top and bottom of the bus exterior and slide the "bill board" into the frame.