In the past my team has used C channel with pins, as it sounds like most people have done. This year we tried to do the same thing but we didn't make our robot out of C channel (first in team history I believe), and we realized the way we wanted to mount it did not meet the rules. Our solution was angle iron on the bumper, and a flange on the frame perimeter. We drill and taped holes into the frame perimeter bumper mounts and drill matching holes on the bumper- bumper mounts. In the end, we had I believe 3/8" bolts
6 10-32 (or 10-24, too long ago 
) holding our bumpers in. There were 2 on the left and right sides of the robot, and 2 on the front (one on each side of the opening). These were one piece bumpers we used. Throughout the season the holes started stripping out so we used flanged nuts on the end of our bumper bolts. Just turn them finger tight and we never had an issue, easy to get off too. With 2 guys, we could get the bumpers are within about 30 seconds. These mounts were strong enough where we could carry the robot by the bumpers, and not cause any harm to the robot.