[EricH]

Flange the lower side of the inner plate, or weld on a small plate that you can put a bolt through.

Plywood is pretty strong, and the thicker versions have a lot of weight loss potential. In 2007, 330 had a very interesting pattern on the underside of its 1/2" plywood base. One of the mentors took a router to the bottom of the robot. He left a support structure for screws and bolts and took the rest down to half thickness or thinner. We had no problems that year with the base.