If you hack some room off the "depth" in the back, you leave yourself some room for a backdrop option, Glassman... Also you could have a little room to sit, talk, set something down for whoever's sitting behind the diorama at any given time. Little table space (and the backdrop would even add privacy if you wanted to just chat with some of yoru online buddies at the show) behind the scene isn't a bad thing.
If I may give my opinion on the dimensions, I think width will display better than depth... Losing depth is just more pallatable from my POV than width.
Consider it like a model train layout for example. They're usually not incredibly deep, but they can fill a display hall's wall with width. People walk along looking at all the spiffy details... That's something to consider for this I think, looking at it from the perspective of someone walking along looking at dioramas and things. Just my opinion anyway.