We didn't build it, we bought it. For symplicity, it flares out from a center "stick" which goes thru the spark plug hole. There are locking sliders on it for mimicking it's demenision in the bore. Kinda like the wind of a subway air vent blowing under Marilyn Monroes skirt. (yes, I am a dirty old man).
Ours works thru the standard small Chevrolet spark plug hole but not always in some of the real trick small motorcycle plugs. So if you have one of these, beware.
It's always best to know your bore and stroke and keep it inyour log book for the class engine size you are running for that meet. Good Luck............J.D.