I cut three 6" pieces from some 12 x 1.75 metric threaded, but you could use 1/2" threaded rod. I turned down 1/2" of the end and threaded it to 1/4 x 20.

I mill up a 1" thick, 6" x 7" piece of 6061 T6 aluminum to form the top piece. You could use a steel plate thick enough to not bend when you apply the pressure required to split the cases. The nice thing about aluminum is that it acts like soft jaws on a vise and will not bung up the end of the crank shaft.

The three holes on the crank case are spaced 120º apart and 2.5" from the crank shaft center. I milled my 3 slots 1/2" wide and 1" long, but three over sized 1/2" holes on your plate would work. I also milled a circular indent in the center .1" deep with a diameter of .63" to steady the bar over the crank shaft that sticks out the left case. You probably don't need the center hole unless you get a real stubborn case.

This is a picture of all the parts that make up this puller.

This is a picture of back side of the puller showing the center recess.

The current price for the puller (part # OB-70602) is $160.00.
The rental price for the puller (part # OB-70602/R) is $70.00, with a $90 deposit.
Prices are current as of 02/04/12.