What about a solid door or a simple piece of plywood? You could place them across the bookcases and secure them with brackets underneath.
Originally Posted by wishfulthinking
I like the idea of using bookcases for the base. Frees up more space for fabric on your other shelves! I've bought two pretty little bookcases from Target. They are 36" tall which seems a good height. If I push them together, my smaller cutting mat fits right on top and it is usable as a cutting table now. But I have a large mat, 24" x 36" so I am looking for a top now. I'd like formica but that is probably more than I want to spend, plus I don't want to add much more height to the table.