Look up fuel bottles and you will find a ton of threads about various options and they all work for oil bottles as well. As for your original question, if you only need a little top off oil and your concern about putting it into the bags is based on normal containers leaking after they have been opened then I reccomend a 250ml nalgene bottle. They are small enough to fit under the seat but I have never had one leak so I just throw it in my top box. My bike consumes a pretty good amount of oil so I carry this one for when the oil gets to the bottom edge of the sight window in 800-1000 miles I dump this in and it gets me to the middle of the window and another 400-500 which is usually enough to get me home without having to buy more along the way. Picture included for size comparison not debate.