Neoprene (also known as chloroprene) rubber is widely used in spudguns where a rubber seal is needed, most notably as the sealing surface in diaphragm and piston valves. It is usually sold as flat mats with a thickness of 1-5 mm. It is a relatively hard rubber material, and can stand up to high pressures. Neoprene is easy to cut to shape, and can be glued to a surface using epoxy or super-glue, or attached using nuts and bolts. If an undersized hole is drilled in the material, a bolt is mounted through it and a nut and washer used to put pressure on the rubber, the hole will be airtight.
O-rings are often made of neoprene.