I would recommend you search for a Blackhawk or Celette bench system. These were popular in body shops in the 1980's. They are out of favor in North America for the body shop market, but are EXCELLENT for fabricators. You can probably buy one for $500. Some are still collecting dust as parts shelves, or are out back. The machined beams function as a surface plate, and the bench can be leveled. You don't need the yellow jigs, just the bench itself and the machined transverse beams. I cannot think of a cheaper way to get a surface plate.