[BeardyMentor]

Plywood, polycarbonate, and sheet aluminum are all going to work well for this. Plywood and polycarbonate are a bit better than the aluminum in my opinion. The reason for this is it makes sure any electronics you mount to it are electrically isolated from the frame. Though, this is almost as easily done with double sided tape or Velcro. The take away from this is, use what ever material meets your budgetary and aesthetic needs.

One thing to note, If you want to use this as an element to stiffen up your chassis, use more screws than you would think. More smaller screws are better in this regard. One 10-24 screw every 4 inches or so around the perimeter works pretty well. Much better than just securing the corners.