its all going to depend isnt it? if theres a passenger then 40 litres of fuel in the back might seriously effect your weight and balance. In my plane, it would almost definitely be out, besides its just a light mesh carrier for most rear luggage, strong enough for two empty jerrys, but not full thats for sure.
If two place and you are on your own, there are ferry tanks (made like those collapsible jerry cans) that you can put in the front seat. If not plumbed in, then use it as a large jerry can. Then you would definitely also want the electric transfer pump skippy was talking about to fill the main tanks when on the ground.