When I built my new darkroom last year I wanted the ability to drop the baseboard for larger prints. What I did was to build cabinets on either side of where I wanted to mount the enlarger (on the concrete wall), leaving enough space between them for my vacuum easel with an inch or so on either side. Then it was a relatively simple matter to attach rails on the cabinet sides into which the baseboard could slide. I also devised adjusting bolts at each corner to perfectly align the baseboard to the enlarger. The top rails are at countertop height and accomodate 8x10 and 11x14 prints easily, and even 16x20 if they are at full frame. Any cropping to 16x20's requires moving the baseboard down to the next lower rails. There is a third set of rails for 20x24 prints though I don't plan to print that large.
Below all of this are some shelves for paper storage.