Increased Fuel Capacity
You really don''t want to be motoring in those conditions. You won''t make much progress and you''ll waste a lot of fuel. 20 gallons is maybe 125 nm. more or less.
For going down the ICW it''s not a big deal to lash 4 or 5 cans to deck. In open ocean you would want it lashed as strong as possible with several strong deck lash points with through bolted backing plates.
I don''t consdider there to be any good way to store fuel cans any place else. Just because someone did and didn''t die isn''t much of a way to think about it. Any fuel below other than inside a real tank is potential risk you don''t require. There enough things to worry about and even a small spill will stink for the rest of the trip and make you feel sick.
A real fuel tank has an exterior vent to deal with expansion. It''s something to worry about in a sealed container.
Given you can''t travel the ICW at night you should not really need much for spare fuel for that leg.