Greetings!
While I’ve never used 80/20, I frequently use metal conduit. With existing hardware & connectors it can be easily used like an erector set.
I have frequently used it to make beds, sofa/beds, cabinet frames, lagun style tables, and much more. By using two slightly different sizes it is easy to make full length and durable hinges. It’s cheap, and since it’s aluminum it shouldn’t rust.
I also sometimes use aluminum flashing for full length corner bracings for cabinets, sometimes just on the inside, with pretty trim on the outside, or sometimes on both the inside and outside corners. Some times a U shaped wrap around for exposed edges as well.
I don’t ever remember seeing 80/20 and they’re myriad of connectors available anywhere locally, and it appears to be quite expensive. Meanwhile, conduit & connectors are easily available locally and cheap.
I’m not dissing wood either, lightweight but sturdy things can be built with wood too, and many people find it easier to work with, probably mainly because they are more familiar with it. The biggest thing with wood and vehicles is to make sure you use screws instead of nails. When using wood, I usually use a combination of liquid nails and screws.
If weight is a major concern, the foamie construction method can be used as well, to make extremely lightweight but durable cabinets & furniture.
Cheers!