Matt,
If you have access to a press brake you can add bends along the edges to make the sheet metal stiff. Your design is using flat pieces of sheet metal. Think of it like a piece of paper. In the flat the paper flops but if you add a bend in the paper suddenly the paper is stiff and can support weight.
Designing the sheet metal to locate and interlock the various pieces is what makes the sheet metal medium a great way to build a robot.
Don't forget your belly pan, mounting points for your manipulator and the bumper brackets. One piece bumpers are the rage.
Here is a shot of our 2012 frame. It's only 3 pieces. Very light and stiff. We use a cnc punch press to make the parts. Takes only 10 minutes to punch all the parts for two frames.
https://picasaweb.google.com/1045493...18256773910530