This 1/8" NPT fuel level switch should be installed inside the container, as shown below. 

This 20-0519 While often not practical to be viewed while driving, this indicator can be used for diagnosing a potential fueling issue.

When fuel level drops, a switched signal can be sent to a unit such as a dashboard mounted indicator or programmable ECU input for safety purposes. The level switch is constructed of corrosion resistant stainless steel compatible with all fuel types. It includes 60cm long 0,5mm2 (20AWG) flying leads.

This switch is only compatible with the Radium Multi-Pump Fuel Surge Tanks (MPFST) and the Radium Fuel Cell Surge Tanks (FCST).  Both of these products have specially machined ports that are labeled "Level Switch."