Yesterday I made a tool for holding the alternator Rotor while torquing the rotor bolt to 95 ft/lbs. I had just hit that bolt with my 1/2 impact, but wanted to make sure it was up to spec (it was close, actually). Anyway, here's a few pics of the tool I made. I used an old brake rotor that I replaced for a friend on a small Yamaha dual-sport. The inner diameter overall was just a bit bigger than the rotor, but I had to cut out the three "tabs" where the rotor bolts to the wheel. So once I did that and ground them down a bit, it just slipped over the rotor with about 3/16 clearance.
Then I welded a nut with a fine thread, for small adjustments of a long bolt that runs towards the center of the hole. It articulates with one of the two holes on the outside of the rotor. I had an old rotor lying around so I did all this on the bench. The bolt is about a 3/8" fine thread, or something metric close to that, I didn't care; but the bolt diameter is just a bit smaller than that outer hole on the rotor.
The procedure is, back off the bolt, slip the tool over the rotor. Line up to one of those holes and hand-thread that bolt inwards just until it is a bit below the edge. I didn't want to take any chance of dislodging the magnet. Because the clearance around the rotor of the tool is small, the other side of the tool prevents the bolt from pushing in any further, once set, if you get my explanation. Anyway, check out the pics. Easy to make, took about an hour.