With the longer National motos, larger capacity fuel tanks can be found all around the pits. Here's a titanium version, found on the Honda HRC CRF450Rs.

A carbon fiber version found on Freddy Noren's CRF450R and all the Geico Honda CRF250Rs.

A carbon fiber tank on the Pro Circuit 250Fs that retains the stock gas cap.

Another that retains the stock gas cap, but on the Yoshimura Suzuki's.

And the CRM version found on the Monster Energy Kawasaki KX450Fs, which uses a flush carbon fiber gas cap.