The Elinchrom Maxilite at 16" in diameter and 45-degrees dispersion is very Magnum-like (13-1/2", 50-degrees) and can easily be adapted for use on the Einstein:

The Elinchrom mount is attached to the reflector with three rivets. Drill out the rivets and the mount comes off. An AB adapter from Jerry Kacey can then be bolted to the rear of the Maxilite once three appropriately placed holes have been drilled in it. I was able to locate an automotive machine shop willing to cut down the diameter of the Kacey adapter so that it could be attached from inside the Maxilite. Outside or inside doesn't really matter.
The Einstein's flashtube winds up properly positioned inside the Maxilite:


As an added bonus, if you drill a 3/8" hole in the Kacey adapter aligned with the Einstein's umbrella tube, a BD-type deflector can be used to modify the light produced by the reflector:

I should add that a grid is available for the Maxilite from Honeygrids though getting one that fit properly was an issue for me:

Dave F.