I am kind of new to this, but I have made my stencils from this stuff at Hobby Lobby, its the cheapest readily available stuff carried in store around me.
I built my vacuum mold from a coffee can, drilled holes in the top and drilled a hole for the vacuum attachment. I found that this stuff works the best. If the lure has eyeballs or fins, you can get this stuff to shrink down to get good detail.
I have found that with thinner stencil plastic and some of the plastic from some toys that the plastic folds down some and the starts shrinking, but not around the lure. I actually melted 2 sets of lures experimenting trying to get it shrink down more.
With the stuff from Hobby Lobby you can still cut it pretty easily, for some of the bluegill designs where the lines are small and close together I used a stencil cutter, the cleaned them up with an exacto style knife.
I am going to try the milk jug though, my kids drink a lot of milk.