Sheet Metal and Bumpers

[nuclearnerd]

I just came across these when researching the same problem on our prototype. Seems like the perfect solution:
http://www.mcmaster.com/#94850a129/=u3dumi

On second read, I should be a bit more clear - if secure the bumpers to the top flange of the outer rails (using the panel nuts), rather than inwards to the webs, you should need any spacers (you might need to make the bumpers one or two pieces though, and have them wrap around the front and back rails.