yea I took one off too and tried to fit it in the locking top part (which doesn't really do anything) and it's too big.  Someone in another 
thread has found one in the spares bag he got fits and works to lock/seal the top.
It was also found that the 1 clear/white o-ring in our spares over the ceramic housing between the negative nut and the mp helps stall if not stop the leaking but both of them, one above the negative (prior one mentioned) and one at the bottom helps it and going strong now for 30 mins for another guy who got a bunch in on coop.
More time with this thing as I may be heading to bed soon so hope you all have luck.  As I posted in the other thread if you check roughly around 1:55 on the video below, which was from TOBECO, you can see them put on the clear o-ring before the mp is threaded on.  I watched it several times then and several times before doing it and never really noticed it.  Now that I have mine on between mp and neg nut it's not leaking, for now, knock on wood.