I have been using the NGK LD05F Spark Plug Cap - LD05F for all my vintage British motorcycles for many years and have been completely satisfied with their quality and performance. Easy installation, durable and weather tight. I highly recommend them in all motor applications.
This cap will serve your needs for water protection. It is a nice tight fit on my son's PW80, which came with a cap that was NOT waterproof. One of the best, most affordable upgrades I could make on that machine!
As per usual, these NGK plugs are high quality, with an excellent fit and nice clean look that goes well with most bikes. The screws you attach the wires to have an excellent bite, once they're on, they aren't coming off.