I just cut out the guts of a spark plug and epoxied a dowel in it. To make a real good one you tap the inside of the spark plug and then round off the end of a bolt and thread a nut onto it. Then screw the bolt through the spark plug that you gutted out and tapped. Set the length you need and then tighten down the nut to lock it in position. This way it is adjustable for different engines.

Another way if you don't have the tools is to buy a plug adapter instead of tapping a plug. For the CM-6 engines buy a 10mm to 1/4" adapter for example and then use a 1/4" bolt inside it. I don't know if this size is available or if you can find one at a hardware store. The only ones I have found is 14mm to 10mm and 14mm to 1/4" This should give you the idea though. Here is a link to a picture of what I mean.

http://www.prcmodel.com/var/files/we.../pg0167_lg.jpg