Here's a photo of my set up. I'm planning on bolting my enlarger directly to the counter one of these days, but it will actaully decease the baseboard size due to the tight quarters I'm working in, so I haven't done it yet.
I do a lot of 8x10 stuff which works great how I have it. It's when I get the head way up high into the joists above that the image hangs off the front of my cabinet there. My guess is that something like rigid plywood could be quickly bolted into place when I was doing this sort of work and then removed to save space. The idea for glass was simply to keep the thickness down so I'm not giving up too much enlargement capability.
Eventually I want to do some even larger work and I'll really have to get creative. I'm thinking about simply flipping the baseboard around and using the swivelling base to rotate the column 180 degrees. Then the image could be onto the floor or possibly onto a stage set on one of the drawers.
By the way, the way I've got it set up right now...the base cabinet is bolted through the left side into the concrete wall very rigidely. The countertop is then bolted onto the top of the cabinet and cantilevered over behind the sink (in the photo) without touching the wall on the right, which is the stairs coming down (vibration).