You need to buy the rubber insert because a gob of silicone squeezed into the drive cone really doesn't work very well. You can make the drive cone out of wood. It is the inside diameter that is critical. Just make sure it fits a standard Sullivan rubber insert.

Borrow a highschool kid and have him turn a drive cone for you in shop class. or you can cut one out with a couple of hole saws. Maple works well, but whatever really strong hardwood is handy down under will do. Saturate it with epoxy and fibreglass the outside if you want it to be indistructable. You can mount the wooden drive cone on a carriage bolt and then chuck the assembly into your portable drill. Use the low speed range and the highest torque setting. I have been successful in starting engines to .46 with a Makita 9.6 volt drill, using the home made drive cone.

My first starter was a wooden drive cone turned from maple and mounted to a 12 volt blower motor I got at a junk yard. It wasn't torque-ey enough to turn more than a .25, but you can do better with an actual starter motor from a salvage yard. Pick one from a smaller car so the motor housing isnt too big for your hands. Get a hefty normally open push button switch from your favorite electronics supply house and mount it to a comfortable spot on the motor housing. Friction tape does well for this.