Used to use margarine containers
The little plastic deadlights were an issue for the surveyor when we bought the boat. Said she would not want to go offshore and have some green water come over on top of those. They leaked anyway and we put translucent margarine containers under them, which contained the leaks and gave us some interesting colored light in the forward stateroom. But eventually I bought 4" bronze and glass deadlights with backup trim rings, at the late unlamented Atlantic City Boat Show. They fit with very little shaving, look really good and do not leak. It's astonishing how much light they let into the space below.