i do the same thing you do - keep gasoline on deck in metal outboard tanks
. i siphon off gas into a container to mix for the outboard - a small 3hp honda
with internal tank. i've used quart oil containers and also plastic soda bottles. for a measuring cup for the oil i use small plastic aspirin bottle. i've also been known to occasionally keep a quart oil container to hold extra outboard mix - i keep it in the dinghy
when making a long trip as insurance
against running out of gas.
by the way, don't know where you are, but in the states one of our beer
makers (coors) is now selling beer
cans with screw on tops. you can actually drink half the can, screw the top back on, put it in the fridge, and the next day it tastes just as good as if you had just opened a new can. i'm going to give that can a try as a spare outboard mix container...