I did run some numbers for flat panel, for my material available in my shed: On two rigth most columns - sandwich panel. Far similar reserve factor or deflection - you are forced to space support 3-4 denser..Make just a 2 ply skin, and the max panel size is pretty small before stresses go too high. So you can add plies or make the skin a sandwich panel. You get strength and stiffness at much less weight with sandwich panels. Well, it does not take much of a sandwich panel to allow you to skip the internal structure entirely.