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.

California residents Prop 65 Warning

WARNING!

This product contains materials known to the State of California to cause cancer and birth defects or other reproductive harm. www.p65warnings.ca.gov