Nothing is stopping us from making one :) It's just like everything else, easier to buy than make (and cheaper too).
We do use the drywall squares sometimes, but in this application the "T" shape of it is not usable. Plus they are only accurate for a few months, and then the rivets start wearing out in the T joint.
I wanted something cheep, quick, easy, and square (yeah right, don't we all!). I thought maybe I should just burn a few out on the plas table, but they warped a bit too much (14ga, and 10ga). Then if I use something like 1/4", I'll end up with 3 guys with back injuries from handling these things. Oh well, we'll keep on doing it the way we are.
Thanks guys.
Mark