Large enough bags that you can hang over the edge of the container and hold with a clip could be better. This would mean that the zip lock seal is not in the water and therefore can't fail.
I bought a vacuum sealer which I find really useful for stopping frost burn on meat in the freezer, so I use it for sealing food for sous vide cooking. I can even put a second seal on either or both ends if I want extra security (not needed so far, even for 6 hour cooking of whole chicken).
It is probably best to either use the freezer zip lock bags or got to vacuum sealing.