A cheaper and lazier version of Mark's idea would be to mount the posters to the foam core and just use four wood blocks (of equal size and depth) near each corner to make them float off the wall. You could drill in to the top blocks at a 30° angle upward from the horizontal and use those holes to grab screw (or nail) heads for mounting, like so:



The anchors might be overkill, but I got carried away...