pic: Conceptual bumper mounts, Universal Chassis

[MrForbes]

Another thing....depending on the game, and how your robot plays it, you might be able to put a full crossmember at each end of the robot, sticking out all the way to the side bumpers, which simplifies bumper mounting.

And the added mounts on the sides near the center wheel don't need to be bolted onto the bumper, they could just be supports that prevent the bumper from being deflected in towards the wheels.