Provide a vent for the tank.
(1) Decide where you will mount the tank and provide a means to secure it such as wooden
cleats screwed/glued to the deck inside the locker. Also provide a hold down strap.
(2) Get an extra cap for the tank you have and
epoxy or 3M-5200 a tube or pipe fitting into the top of the cap after drilling a hole.
(3) Mount a vent fitting throught the hull just as you would for a permenantly mounted inboard tank and connect it to the tube/pipe on the cap you have modified with a hose.
(4) Make a place to store the normal cap for the portable
fuel tank such as: Tie a little net bag to the portable
fuel tank and stick the normal cap in there when the tank is mounted in the locker.
(5) Go sailing/motoring.