Those plastic cans you can buy for five bucks aren't designed to be fuel
tanks.  I have a six gallon marine fuel tank with vent and pickup tube
certified for motorboat use.  It lays flat in the top shelf.  Most outboards
use a siphon pump to get the fuel started towards the motor.  You can use
one of these with a long fuel hose to add to your tank directly without
removing the marine tank from the rack.  Yes it weighs 50lbs full BUT if
you're prudent, when you burn off the first six gallons or so, just reload
from the top tank and then it only weighs about 5lbs.  So you actually drive
with the upper 50lbs only 100 miles or so.  The fuel fitting is a quick
disconnect with a valve in it so it makes refueling simple.

pensioner