I made one by taking an old spark plug and busting the porcelain out of it leaving a hole down through the center. I then epoxied a dowel (big as the hole) into the plug cavity. Let the dowel stick out of the plug some 3/4" into the engine. Just guess at the length. On the outside it doesn't matter if it is flush with the outside or sticks out some.

The object of using the piston stop is to stop the piston before it gets to the top (maybe even about half way up the cylinder wall so your degree reference each direction will be most accurate).

There is nothing new about using a piston stop and swinging the piston up the cylinder wall from both directions to find the degrees of crankshaft rotation each way. I read about it in the 50's on hot rod car set ups and I'm sure it was not new then. There is also nothing new about feeling for top dead center with your fingers with a single or twin cylinder engine (not as accurate but good enough for our purposes).