It can't be that difficult. Turbo manufacturer's are constantly tweaking their designs to improve performance. And each tweak requires a new mold and tooling.
I have to assume that at this point in time, all turbocharger designs are created in a CAD program. And I can guarantee you that it is incredibly simple to mirror a design in any and all CAD programs. In most cases, it's literally one click of the button.
So, if they're already creating new molds every time they tweak a design for better performance, why not create a second mold that's an identical mirror of the original. No additional design has to be done because it will flow exactly the same.
That's all I was getting at. At this stage in the game, you're absolutely right that it would cost a crapton to have one custom designed by a company that doesn't already make one. I just find it strange that turbo manufacturer's would refrain from creating a symmetrical system. (See:
Nelson Racing)