Hi Princo,
What is the proposed size of your outdoor ship? I think you will need a proper shape (google series58 generator), a slenderness ratio of at least 4 to 5, and you will need ballonets. Week PU films might forgive a volume change of the lifting gas due to a change of ambient temperature and pressure, but they can't maintain the shape and can't withstand bending moments in gusts or at higher speed.
Best, Johannes