[Otaku]

There's two good ways to do this that I can think of.

The first is if you have a reasonably solid frame, 1/4in thick or more, solid bar chassis (like 675 uses... a lot...). You could have matching holes in the frame and bumpers, tap the holes in the frame (or maybe weld on matching-thread nuts to the inside of the frame, assuming you're using weldable materials). Then what you do is get bolts that are just longer than the combined thickness of the frame member, nut if applicable, and bumper, and use that. Installation/removal is quick and easy, especially if you've got a socket driver on a power drill.

The second option should work for pretty much any frame. Pins. Do the same thing as above; holes in both the bumpers and frame members in matching place, or you could press or bolt in the male pin to the bumper. You can use either bolts or machined pins. Bolts will be cheaper but harder to drill a hole through. I really suggest using a drill press and a vice for this. Trying to drill something small while holding both things in your hand... bad idea.

Anywho, so what you do is once you've got your holes drilled and your pin ready, you mock it up on the chassis, and mark where to drill your hole. pull it apart, drill the hole, and put it back together with a cotter pin through the hole. For extra strength, put an appropriately sized washer in between the cotter pin and the frame member.

I can't guarantee either of those will conform to FIRST rules (depends on how you design them, honestly), but that's on you. Option #2 will be quicker to get on or off. Both are pretty quick though.