You might not have seen
http://www.chiefdelphi.com/forums/sh....php?p=1500810, which is a good bumper overview.
Teams use a variety of attachments -- some have brackets on the bumpers, others put
T nuts in the bumper boards and use
a knob with a bolt in it on the other side. You can also put the bolts in the bumpers and have the nuts inside the frame, but you need to pay attention to clearances during bumper installation/removal. If you're planning to have an attachment that comes from inside the frame, ensure that you drill holes prior to drive base assembly and have access in the area you're planning to attach to.
If you go the bracket route, quick attachment methods include cotterless clevis pins with a ring in the top, or
safety pins. A thicker bracket could be tapped and then you could use the knob above to install it.
When deciding your attachment location, make sure that you pay attention to R22, which specifies the vertical location of the bumpers. It's a common practice to allow for a bit of sag of the bumpers by setting the lower edge of the bumper somewhat higher than the 4" minimum. You'll have less trouble with sag if you avoid stretch material for the bumpers.
You should be able to swap your bumpers in 60 seconds, ideally without any tools.