Sniper,
A fuel cell is not much more than a fuel tank with a ballistic liner (sometimes the liner is the fuel tank?) and fittings to allow fuel flow into and out of the tank. Foam filled cells use an open celled foam to keep slosh down to a minimum (could do similar job with baffles), but the foam can be removed to get into the fuel pump and other components if they are mounted in the tank. Be sure to get a cell/foam that is resistant to alcohol based fuels and gasoline since there is a reasonable alcohol content to California fuels at the pump.
18 to 20 gallons will bet you a couple hundered miles depending on how much you use the throttle...
I have just built a 10 gallon cell for my project in order to make the shape fit the confines of the car and keep the weight where I want it (6 pounds per gallonish for gasoline).
Most off the shelf cells come as a rectangular box with fittings for pick-up, return, fill and vent.
Later,
Steve-O