I made my own on the first one, I knock all the center porcylen and center electrode out of the spark plug and inserted a long 5/16( I think that was the size) bolt with a nut on the cylinder side to lock and hold the bolt into plug body and used it as my first piston stop with the head on.
You can buy them from most any tool truck, they are usually brass or bronze mix also which is good
one of the ones I've bought had a small hole in the center to relive the pressure when moving the piston up towards TDC, I think it whistle also