1/2" UNISEAL Flexible Tank Adapter (bulkhead).
Plumbing through tank walls is made much easier and less expensive using a Uniseal®. Unlike bulkhead fittings a Uniseals can handle slight irregularities or curvatures without leaking.
Feel like rubber, but they are made of DuPont Alcryn® rated to 40 psi and warranted for 25 years.
Instructions
1-Cut hole with holesaw to the size indicated.
2-Ensure that the hole is cut clean. Irregular holes could cause poor seating and possible leaks.
3-Insert the uniseal with the wide surface facing out.
4-Make certain the pipe to be inserted is cut clean and square, file or sand the edges so there are no rough or sharp edges to cut the uniseal surface.
5-Use liquid soap or teflon lube on the pipe to be inserted. Insert the pipe into the uniseal as far as needed to clear the inside of the seal by about 1". The lubricant will be squeezed off of the pipe as it passes through the uniseal.

For 1/2" pipe (FITBG00.5) = 1.25 Hole Saw