I just changed out the water pump on my 98 e320 and ended up using the larger of the two wrenches in the fan clutch wrench set you can borrow from Autozone. it was sized such that it would slip over the four allen head bolts and grab them enough to loosen the fan clutch nut with the 36 mm. It worked for me.
I had initially made my own holder out of simpson tie strap I had laying around by drilling holes and using my 4 in grinder to shape the edge. Unfortunately the strap was not quite thick enough to withstand the force and bent too much to work properly. However, if you find a strap thick enough, making your own would work if you don't have access to the proper tools.