If I knew how to weld, had a welder at my disposal, and had lots of scrap metal (that didn't weigh a ton) laying around, then I'd probably build my own as well.

As it is, I'd love to learn how to weld, but the cost of everything to get me started (note I didnt' say proficent) would probably supply me well over a dozen stands of my choosing.

As to making your own harnesses... when I used to repell a lot all we ever used was our own hand tied Swiss Seats. Long as you tie it right, no problem. Not exactly comfortable though.... I'll stick with my vest!