Miguel,
The stretchy film as a mold covering will probably work fine as long as your tanks don't have any concave surfaces. If there are concave surfaces you will have more luck covering the molds with a low temp iron on covering like Econokote. That is what I used to make the molds in the thread I posted below.

The nice thing about the Econokote covered molds is that they can be reusable. Those particular molds have now made three sets of tanks and are still in good condition.

The stretchy film for holding layers in place is a good idea too.