I'm building a cargo drawer system using 1/2 x 1/2 .016" thick wall and the whole thing is rather light!! The drawers are made of the same material and will be lined with the thinnest material I can get my hands on.
There used to be a company in Oklahoma that made running boards for some models of SUV that had a removable lid, and you could store stuff inside. I had a Dodge Ramcharger equipped with these, they were thicker and stronger than normal.