[EricH]

1/8" Al, as wide as the gearbox and a little bit longer. Use to mount to a member parallel to the gearbox. (At least, that's what the IFI Kitbot gearboxes used, and they were plenty strong!)

Before giving advice, I'd like to see your design. If you have any pictures of the robot's frame before the reinforcement, after the first reinforcement, and after the second reinforcement, and can post them, that would be really helpful.

What the local engineering-type folks will do with said pictures is offer constructive criticism (well, we hope it's constructive!) to help you figure out not how to fix the problem, but why the problem happened and how to prevent it.