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We always mount our bumpers on 90º angle brackets directly to the top of the c-channel using quick release pins. When we use this system, we can change bumpers in around thirty seconds by simply pulling the pins out, kicking the robot to make the bumpers fall off, line up the other set, and slide the pins back in. The pins are strong enough to keep the bumpers on the robot even when it's upside down (we use the variety with a ball bearing at the bottom of the pin), but easy enough to remove that they're not too much of a hassle for us to hate changing them.

EDIT: These are the pins we use.