In the US every year 147 Million Gallons of Fuel are lost due to evaporation in the vehicle's tank. This simple design may help to reduce the surface area of the fuel in the tank at all times, thereby reducing the rate of fuel-evaporation.
A floating plastic plate on top of the fuel inside the fuel-tank, as shown in the diagram, will reduce the effective fluid (fuel) amount in contact with the atmosphere, thereby reducing the loss due to evaporation. The only opening in the centre is to allow for fuel/air entry as needed into the tank. Other openings can be had to allow for the fuel-pump, fuel-level sensors, etc. The cost of putting in such a perforated plate inside the fuel tank is very minimal for the vehicle manufacturer.