Last year I read about a method of attaching a strap to a SureFire 6P flashlight which allowed full use of the light in the isometric "Harries" position while not impeding mag changes, etc. The company didn't provide any helpful pictures on their site, but a solution (maybe not theirs, but a decent-enough approximation) came to me after a few more beers.
I dug up some Ideal #12 hose clamps and a piece of 1" nylon webbing, and tried it out.
It works! And the whole project took about fifteen minutes, including the time spent rummaging around in my toolboxes.
The light is placed along the palm of the weak hand, end-cap switch by the thumb, and the weak hand then goes wrist-to-wrist (watch-face to watch-face) with the strong hand holding the gun. Try it out, and then tighten the strap to your preference.