The end piece solution for vertical stacking sounds easier in a way, if you have machining available; I don't. I can cut, weld, finish, and do other metalwork, but CAD/machining I can't easily do.
My local metal supply houses are more on the industrial scale; thank you for the Openbeam suggestion. That's already led me in fruitful directions. Maker stuff. Makes sense. Didn't really think about it.
I tend to use a lot of salvaged/recycled wood and metal materials, so off the shelf extrusions aren't something I typically look at; will do.
This is giving me other ideas.