Thank you for asking questions. Half of throwing ideas at a wall is making sure the wall will bounce them back
Used: I hadn't thought of that and I don't know where to get used tube/pipe.
Aluminum: Generally more expensive and not immediately available at a hardware store. Though I could be convinced to go to a local manufacturer to get a quote. Imo for a 4500lb truck, in my case, the weight of the tubing is a smaller concern than cost and strength. I'd save maybe 10lbs maximum in the switch. Some math: If I'm lucky I'll get a 3-4mpg increase, so a reasonable hope is +3.5mpg. With baseline 19mpg and $2.20/gal ave. in my state, I'd save 1.85 cents per mile. So for every dollar I spend it takes another 55.6 miles to break even. At my budget of $140 (most is for a good polycarb sheet), which I'm already probably going to break, it'll take almost 8k miles to break even. Damn northern oilsands making gas too cheap to properly ecomod
Glued and riveted internal lugs: I hadn't thought of that. I'm not very familiar with this method. I had considered riveted connections when I was looking at EMT. What are the advantages? Screwing something in then gluing it seems easier and perhaps stronger.