I spotted 4 original Champion spark plug caps for sale on eBay at a reasonable price and thought that they would look good and suit the period of my MGB.
When they arrived i realised that they would probably not be quite so watertight as the ones i already had. There is quite a large gap around the inside of the cap and the plug stem ….. plenty of room for water ingress on a wet day.
The original plug caps fit very tightly around the stem of the plug making a good watertight seal ….so I cut the ends off the original caps which fitted nicely up inside the champion caps which made a good seal around the stem of the plugs.
On the other hand i doubt i will be doing a lot of driving in the rain so should not be a regular problem anyway!