This is not my invention, I read about it a number of years ago in some publication, but it is so simple and easy and looks good.
Machine a brass cap to snug fit on the spark plug cap and solder your spark plug wire to it. Slip a small O-ring on the wire, and a larger one on the brass cap near the open end. Plug the opening in the cap and dip the thing in that rubber like compound that you dip plier handles into for grips. You can dip a couple times to build up as desired. Then with a knife trim the hardened dip around the plug and and pull it out, you have your very own custom boot.