Manways and covers are typically constructed from aluminum, cast-iron, or a composite material such as carbon-fiber.
They are designed to withstand the heavy loading of transport vehicles and protect the spill containment bucket and fill from dirt and debris.
They can either be an integral part of the spill containment bucket, or they can be separate components such as the type used for below-grade spill containment systems.
As can be seen from the lower image on the left the cover is painted a color that represents the product stored in the UST. This color must be in accordance with the requirements of the API recommended practice RP 1637, and COMM 10 code section 10.230(12)