[DonRotolo]

Make sure there are places at the top edge to mount superstructure (i.e., manipulator), perhaps turn the top edge of the inner rails inward. Maybe make them a little taller too, so the top is just above the tops of the wheels - your manipulator team will love you for giving them a perfectly flat surface the size of the whole robot (your maintenance work will be from the bottom looking up). Also, count on using your bumpers as structural elements, which means only the mounting points on the outer rails need to be strong, the rest can be weak because you have that 3/4" plywood holding it all together. For the bottom plate, consider fewer and smaller lightening holes, as mentioned above so you can easily mount your electronics and gear there. Last, line up the shift pistons and use just one to shift both at the same time. There's a thread here somewhere that shows how one team did that.