The link doesn't seem to work.... were it me, I would take apart your current gas cap assembly and use the bolt pattern from that to build up the plate for the handles.
If push came to shove, a competent machine shop should be able to weld in the holes in the pattern that fits another style of tank, and drill new holes where they need to be to line up with your tank.
It all comes down to how professional the results need to be. No two passengers are going to put their hands on the grips exactly the same way. Arm length, shoulder spacing, hand size, glove thickness, how snuggled up they really want to stay to the operator.... the amount of rotation of their wrists....
I'd want to mock it up in rough form, and try it out before I committed to a design, to be honest. The more I think about it, the passenger almost needs a set of grips at the same sort of angles as the rider's controls, just elevated high enough to match the ergos of the rider's set up.
