You dont have to use cotter pins. Truth be told, they can get a bit expensive to buy. Someone else on here came up with a great idea to use hair pins (aka bobby-pins). I think its a great idea, because you will lose these things when youre playing with them. And bobby pins are dirt cheap and its not going to be the end of the world if you lose one.
Glad that worked out for you. Just be sure to wash that thing out before you make a grenades with it! Wouldn't want any weed killer getting stirred into the mix now would we.... or would we...?!