Open or close portable fuel tanks during storage at anchor

[Learning Curve]

Hi All,

Just wondering what's best practice here. I have 2 x portable fuel tanks (approx 12litres each) and both are now stored in the same locker. My question is whether the breather on the cap on the containers should stay open when I leave the boat all week or whether the breather should stay closed.
Logic tells me to close the breather as fuel will evaporate, and also create a potential safety issue with fumes but then is it safe to leave the tanks in a locker in the cockpit where they expand and contract and look to spew out fuel anyway. I'm really not sure if the breather should stay open or closed when not in use...and how tight.

I have been leaving the breather cap closed but not tight

What I don't like is coming back to the boat to find petrol stains all down the top of the containers that I have to clean up and they wreck the lockers with a build up over time. Any guidance here. PS: The locker door doesn't have a vent on it and i was thinking of adding one to help.

I've attached some pics of the old tanks that were left and I've replaced (same type) and you can see the black build up in the locker in one of them ...again from previous owner.

Summary
1. leave fuel container breathers open or shut and how tight when not in use?
2. Is it a good idea to install a vent in locker door in cockpit?
3. Any other advice here.

Cheers