Looking at the design, your CG is too high and too far back to do much about.
I would suggest creating a wheelie bar. Hang 2 PVC tubes from the back-top of the robot (full height of the robot) so that the will swing out with gravity when the robot tips. Connect them together with a 3rd piece of PVC for structure (box frame). Put rubber boots on the ends of the 1st 2 pieces of PVC so that it does not slide on the diamond plate). use string of some sort to prevent the wheelie bar from going too far.
It wont look pretty, but it should be effective. And sorry I am late in posting, I wasnt on all weekend
