Don't discount wood as heavy. I used 3/8 Baltic birch plywood for my checkbox because it is lighter and uses less hardware than 1/8" aluminum. I would feel completely safe using that box as a jack stand in case of emergency, so with the right design, strength is not an issue either.
As an aside, I also think the 3/8 was too thick for some of what I did. The next version will have a couple of 1/2" parts, but be mostly 1/4".