First, find a diesel can that's not Nader-ized and will actually pour fuel. Or drill a hole in whatever you bought recently and add a sheet metail screw for the vent stop. Bas . . . ds. I've spiled more freakin' fuel trying to pour stuff out of the government improved fuel cans than I ever did with the old ones.
I use a 1/2 gallon round plastic jug that hummingbird nectar came in from Wally World (yeah, I'm a geek). It was what was on hand when I needed a funnel more rigid than a milk jug and bigger than any I had on hand of the usual automotive type.
It works well. The handle wedges nicely and keeps it from moving about and it's rigid enough to pour into (with appropriate can mods). Just look for a wide mouth, round plastic something with the most rigid plast construction you can find.
Alternatively, you can always hold a stupid-can above the tank and siphon.
-WSJ