Making custom water tank to fit under main cabin bunk, making as large as possible to give the largest volume of water possible

Making the panels up to make it into a plug to make the water tank as largest possible size

Making the end panels as deep as possible to get the largest possible volume

The long panel to fit as close as possible to the shape of the hull and at the front end sit on the floor bearers 

Should be large enough to go on long voyages 

The top brace to keep the shape while fitting the curved panel that will round the curved lower face of the tank the only part that is going to be a head ache will be the area where the bottom of the tank changes direction.