One technique I have used before is to dry mount the each print on heavy illustration board cut to the same dimensions as each print. Then cement 3 corks to the back of each board to hold them away from the wall. The shadow that each casts become the print's frame. The corks are arranged as one at each bottom corner and one in the center of the opposite side. The corks should be far enough away from the edges so that they are not easily seen. This gives a three dimensional quality to each print.