I'm going to assume that you are using a microwave to heat your plastic. You will get small bubbles in the plastic especially if you heat the plastic all at once. Try heating in stages(2mins, then 1 1/2 acouple of times). Also you have to stir the heck out of it after each heating. This should reduce the bubbles.
If you have humidity in the salt you will definately get bubbles, big time.
I take my salt and warm it on my hotplate to dry it before I put it in the plastic.
The other thing is practice and eventually you will find what works for you; everybody pours a little differently.