I sort of looked into this. I don't know how to weld, so that was a barrier. That said, going to HD and picking up some steel will not be a cost effective approach. You'll probably want to go to a scrap yard, much more choices and cheaper.

I looked into 80/20, and I priced out a DIY rack similar to the prinsu, with more crossbars, and an extruded side rail with Gamiviti mounts.

I ended up purchasing a prinsu rack instead. Honestly, the Prinsu is great, its lightweight, holds my RTT plus gear safely, and its adequate. If i were going to do this again, I'd probably go more the DIY 80/20 direction. tnutz.com is MUCH cheaper than the official 80/20 online sites. And while the prinsu rack is great, I know a heavier, thicker DIY'd version would have been a lot stronger. I'm positive "stronger" isn't necessary, but the prinsu rack DOES deflect slightly. I like things that are overengineered to the point of ridiculousness.

Look up gamiviti DIY roof rack mounts, took me quite a while of research before I tracked those down, but they looked super easy to use in different rack applications. If I ever do a DIY rack for a 4runner, it will probably be a combo of welded aluminum, 15 series extruded aluminum, and gamiviti mounts. Be aware, unless you can weld yourself, you're probably going to end up still spending quite a bit of money. The aluminum options I priced out all ended up over the cost of a prinsu rack. I'm pretty sure welding an entire roof rack together is NOT an afternoon's worth of work.

Hope some of that is helpful. If it wasn't clear, I'm actually very happy with my current rack choice, it was a good choice, very cost effective (one of the cheaper rack options out there) and its well built. But its not perfect, and this is sort of where I've landed if I had more time, to do the best version of a roof rack for me.