I got an old piston and drilled the crown, covered it in gritpaper and fastened it to a long piece of 8mm stud with nuts holding it and the gritpaper in place.
put stud in a drill
I put a mastic tube nozzle in the spark plug hole to protect the threads and keep the stud centralized and spun away.
I even put a small circle of grit on the crown first so the top grit was more angled than the piston and gave me the perfect squish.
I still blew it all up