So I am going to head to the showcase and try to explain to the sales-people this problem. I know the solution is expensive for Techo, and that they are going to tell me to use another product for square corners, but Techo is not the only manufacturer with this problem. Perhaps pointing that out will open their eyes. They need to understand that some of us smaller contractors can't force our customers to change products, that might cause us to lose business.
Tomorrow I will post photos of the job in question.