cheap... are you handy with a saw? Go to the hardware store and buy lengths of thin flat wood, cut it so that it go around the outside of the painting and tack it with nails to the side of the canvas. You don't have to go crazy and make angled cuts for the edges of the makeshift frame, just cat flat pieces of wood and butt them up against each other. This way if all paintings are the same size then they can be standing next to each other and not touching each other because the wooden slats would be protecting the painting. Did that make sense?