For quick and easy swapping of bumpers, nothing is easier than using using studs and hairpin cotter pins. 228 has used this method since 2009, experimenting with different methods of alternately having the studs attached vertically to the robot frame or having them attached horizontally to the bumpers, depending on the bumper configuration.
As long as you don't lose any of the cotter pins, you can swap bumpers in 20-30 seconds.
Here's a photo of 228's 2011 robot with the bumper mounts in the background.
