[Qbot2640]

We typically use 1/4" Baltic Birch plywood - very strong due to solid, consistent thickness ply...but with your demonstrated failure I would use 3/8 and reinforce it with some aluminum straps or angle if possible.

Is there no way to make your frame a complete perimeter? Then the belly pan only really needs to add stiffness.

If you are not pushing the edge of the weight limit - you can go as high as 3/4" Baltic Birch - and that would withstand a great deal of stress. Plus weight down low is a good place to have weight!