I keep 8 yellow diesel
5 gal containers in the aft cockpit
. I've always filled the tank at sea from these containers. I lock the twist valve on the spout, tip the can upside down into the fuel filler tube, the valve opens automatically when the spout is inserted, fuel flows without any spill. I had an old style container and hated it because the finger over the spout trick doesn't work
well when the boat
is pitching and rolling. I gave it to a guy who preferred old style to modern containers.
RE OP post, I have no problem with oilcanning on the cold N California
coast whether containers are full or empty.
Before I learned how to use the spout I used a shake siphon. It works but it is slow.