You have seen the original to know what it looks like, right?
To build one, the big design parameter you need in mind is it needs to act like a straight extension. That is, the end that will recieve the torque wrench must be in a straight line and centered directly above the bolt head, thus keeping the center line of the wrench perfectly perpendicular to the center axis of the head stud. If that parameter is met, you can make the offset part any way that suits your fancy, so long as it is rigid enough to maintain that positioning during the loosening/tightening process.
If I were doing it, I'd start with a 12 point box end wrench and work up from there.