We make and test our 19 gallon aluminum tank in-house.  It costs a bit more, but the design is exactly what we want.  Side baffles (green here) with one-way doors (red) keep the fuel from sloshing side-to-side while encouraging the fuel toward the central pickup.  In addition, a transverse (gray) baffle prevents fuel starvation under hard accelleration.

All the connections from the tank to the filler are also included, and installed at the factory.

Tank cutaway
Fuel filler
Capacity: 19+ Gallons
The tank is supported by flanges that bolt directly to the main frame rails while still offering some flex if the car is in a collision.
The filler pipe is extra heavy (it's just behind the tire) to resist stones thrown by the tire, and the air vent tube allows the tank to be filled completely.
A fuel cell for racing is also available.