Made my own version of a top dead center indicator today using some of the examples I have seen on the forum. I had a piece of steel rod that just fit in the spark plug base after removing the insulator. I drilled a 1/4" hole through and then epoxied the guide into the plug base. Cut a piece of 1/4" SS rod to length and epoxied a washer to the top to keep it from falling in the chamber when the piston is down. Seems like it will work just fine.
Here is the tool and then a photo of it installed and the piston at TDC. (edited)
