for the past couple years we have used C-channel on the bumper itself and it would fit on the robot either on square tubing or another set of C-channel. the bumpers are held on with pins like these:
http://www.homedepot.com/h_d1/N-5yc1...atalogId=10053
the pins are easily removable and have held very well. usually 2 pins per bumper part. when i find pictures of the back of our bumpers i can post them for you.