Go to your local liquor store, and wait outside for a drunk to come out with one of those tiny little plastic bottles of liquor. It holds 50ml. After the drunk finishes off the bottle in one gulp, he will toss it, so you take it, wash it out and fill it with 2 stroke oil.
Dumping a full 50 ml bottle of oil into a full 2 L gas tank will give you a ratio of 40:1 (2000ml/50 ml=40)
If you don't fill the gas tank all the way, then that's better, your ratio will drop. If you put in 1.5 L and dump in 50ml of oil, your ratio is 30:1.
I collect these bottles, clean them and mount them on my builds as an emergency oil bottle, so that you can fill up at any gas station, and toss the contents of the bottle in, (of course, do all this with the petcock closed) , shake, an be on your way. When you get home, refill the bottle with oil and remount to bike until next emergency.