I've had good luck with the NUK bags (I have frozen in 2, 4, and 6 oz portions). Thick plastic, double zipper, seams have never split on me. I've only had 1 leak so far (in 10 mos). They are also a great value ($10 for a box of 50 on Amazon). I do thaw mine in a bowl or dish so that the milk isn't lost in the event of a leak.