We had a 1x1x1/16 aluminum tube that went around our frame perimeter. The bumpers had 8 aluminum U extrusion pieces 2 inches long that were mounted inside pockets on our bumpers. This allowed us to get a tight fit between the U channel and the 1x1 tube. 1/4 inch holes in the 1x1 and the U brackets allowed for a quick release pin on each bracket
http://www.mcmaster.com/#98470a135/=12cpcnp. To put the bumpers on or take them off was about 30 seconds.
My explanation might not be the greatest but once we get our crate back from fedex I can post some pictures.