-very long straight edge
-lots and lots of tape
-And as much cardboard as you can find.
Step 1. Take measurements off of your meticulously drawn diagram and change them, somewhat arbitrarily, based on your uneducated guess of what will fit through the companionway. Measure those new somewhat-arbitrary lengths on some cardboard and start throwing tape down.
Step 2. Add another side, but make sure it is the mirror image of what you actually want, so the entire exercise becomes somewhat arbitrary.
Step 3 - 16. Continue using somewhat arbitrary measurements and miles of tape to make a flimsy cardboard mass roughly the size and shape of your old watertank
Step 17. Add more tape.
Step 18. Carry it, awkwardly, to the boat and attempt to shove it down the companionway without collapsing the flimsy shape of the cardboard mass. Get frustrated, and forget to take pictures.
Step 19. Place it, somewhat arbitrarily (since it's actually a mirror image of the shape it should be), in the open space where your water tank used to be.
Stare at it.
Scratch your head.
Ponder trying to fill it with water.
Go to bed, feeling somewhat arbitrary.