I would look up a local granite counter top fabricator and grab a remnant from him. A lot of time homeowners will select slabs for kitchen/bathroom counters that leave significant remnants that the home owner can do nothing with. The fabricator (at least ours) has hundreds on pieces of granite and quartz that people will never want. I'd go check that out and customize a bench to accommodate the slab you can get. They're dead flat and you can just scrape up glue. I've gotten large pieces for less that $50.....