Caps are available for a variety of applications to fill openings in underwater camera housings.These are made from durable impact resistant plastic. There are caps for port openings, covers for ports, and caps for viewfinder holes. Whichever cap you need it will provide protection from dust and debris from fouling your equipment. They are not watertight and only serve to protect the sealing surfaces to cover the opening.
Plugs are for plugging holes in underwater housings and are made of plastic or aluminum. They are also watertight and you can pressure test with these plugs.
The focus knobs are pushed on over the top of your housing's existing focus knob. This will give you the fingertip control you need when manually focusing on your subject.