I cut a thin plexiglass panel and attached it behind the front veneer with some clear epoxy. By clamping the surfaces together I was able to stop the front veneer from warping, but I definitely lost some definition and brightness from the display. In a future version I'd like to experiment with other methods of stabilizing the veneer.

I also installed magnets on the inside of the box; these matched opposite polarity magnets on a wooden construction that holds the tablet in the right position against the front panel. This way the display is exactly where I want it, it's easy to remove if I need access to the tablet, and it snaps back in.

I also epoxied some picture hanging hardware into the slots I made on the back of the concrete cast; they're sized to fit a normal screw head so the shelf can hang with a few screws and a wall stud.