The ideal enlarger work surface is a block of granite of the appropriate dimensions. Chiselling out the shelves, however, can be a real bear.
Short of that, I agree with the other's suggestions of using commonly available components, but maximizing rigidity and stability. For example, 3/4" plywood makes a nice tabletop - if the front and rear edges are reinforced. I glued a 1x2 along the front edge of mine to accomplish that, and the sides and back are screwed to a frame to provide the rigidity.