Have in a medium-sized photo (under 18 x 24) mounted on thin black Styrene, which is around 1/6" thick. Customer wants to frame with glass and spacer (no mat). My usual thought would be to stick it to foamboard (or gator) for added rigidity. For those of you who have worked with Styrene mounted photos, are they sturdy enough to just back with loose core board, or do they need to be mounted?