Never use plastic containers as you cannot earth out the static electricity. When pouring fuel into the tanks, you could loose the aircraft through static build up.
Do not vent the jerry cans, as it is not necessary. If fumes were to find there way into the cockpit you would be a flying bomb. Just check they don't leak before securing them into the aircraft.
If you have to carry extra fuel as suggested, good quality jerry cans, secured upright are your "safest" option.