UNISEAL Flexible Tank Adapter (bulkhead).

Plumbing through tank walls is made much easier and less expensive using a Uniseal®. Unlike bulkhead fittings, Uniseals can handle slight irregularities or curvatures without leaking.
Feel like rubber, but they are made of DuPont Alcryn® rated to 40 psi and warrantied for 25 years.