Here is another simple idea for making spark plug caps.
Many of the Chinese 40 TPI spark plugs, we all use, have a 4mm terminal. For these spark plugs, we can use a female 4mm bullet connector and some heat shrink tubing. I soldered the plug lead wire to the outside of the bullet connector and crimped the tabs over.
I used standard 5mm heat-shrink tubing. A short piece over the crimp joint and a second, longer piece to partially cover the spark plug insulator at one end and to form a bend relief at the cable end. The bullet connectors are made from hard (and springy) brass which does not like to be bent through 90*. So the bullet connectors must be used as straight plug caps. A permanent bend can be formed in the heat-shrink bend relief if it is formed before the heat-shrink has cooled.
Here is one I did earlier, fitted to my Westbury Seal.
Neat and inexpensive and they seem to grip the spark plug well. Of course, you can always tighten the grip with a pair of pliers
Cheers
Mike