If you are using the kit frame material, I highly recommend the bumper brackets that are sold by AndyMark. They slide right over the frame rails and you drop a hitch pin or bolt through the holes. About 30 seconds to remove or replace a bumper. They are not cheap, especially if you are making 2 sets of bumpers, but are worth the investment.
The same technique can be used for non-kit frame rail materials, if you have access to a metal brake or a sponsor that can bend you some. One of my teams did that a couple years ago with great results. Or find some channel the right size and use that. That can also be done with the kit frame.
I'm not a big fan of permanently attached bumpers, I strongly recomend a solid, well thought out method that allows them to be removed and replaced easily. Keep in mind that the rules recommend that one person should be able to do the task in under 10 minutes. Also keep in mind that the bumpers have to be off the robot for weighing and sizing. You would not believe the number of robots that it takes an hour to try and put the bumpers back on.
When I give seminars, I emphasize that the bumpers need to be part of the design from the beginning, not something that is hacked on to the robot on ship day or at the regional. It's very obvious when inspecting a robot if this was the case or not. If you pay attention to what you are doing, that 3/4" slab of plywood can provide a lot of structural integrity to a lighter weight frame.
Also, if you are only going to have one set of bumpers, and looking good is important to you
, making slip covers for each bumper that attach with snaps or velcro and fit snugly is strongly recommended. The elastic covers that were available last year came loose or got pulled off due to robot-to-robot interaction. Saw some penalties due to the bumper cover hanging down below the bumper zone. Duct tape and/or cable ties are not an acceptable method of trying to hold the cover back on. I know it's a pain to have to store them in the pit, but I recommend having two sets so you don't have to worry about covers coming off.