My suggestion is a piece of etched glass for the back. Or if you wanted get some tounge and groove and use that for a back. Pull the clamps tight and measure the X (corner to corner) to make sure its square. Coat the thing with 6 coats of a clear (spray is best) sanding in between starting with 320 grit then working down to 2000 or something like that. Gives the finish some depth makes it look very high end. I can rattle off ways to do it for you for days but to each his own. My best suggestion is get some form of a back for it the wood would match it best. If you want further suggestions directions or details let me know. Beyond that looks good make sure your joints are pure and if its square to start it'll be square to finish. Glue is better than nails.

Ryan.