Was chatting with RC last night and we also came up with cotter pins + clevis pins as a cheap alternative to another solution. You can just buy steel clevis pins from mcmastercarr since the weight goes with the bumpers and for the most part bumpers seem to be underweight.
http://www.mcmaster.com/#clevis-pins/=57qrqv
Then just pick your style of cotter pin to secure the other end
http://www.mcmaster.com/#cotter-pins/=57qrx2
Now for the expensive solution.
http://www.mcmaster.com/#aluminum-qu...e-pins/=57qs26
Quick release pins, push button yank out.
Downside is they are about 20 bucks a piece though theoretically you need just 1 per bumper right in the middle of the span if your bumpers are secure enough they shouldn't be able to bend up at the edges if they get yanked.