Like this:
"Stretch a polypropylene film of ... over a ...."

Example:
"A special group of sealing and wrapping films is used to cover and seal laboratory dishes, glasses, Petri dishes, biological culture cells, and other containers by stretching the film over the vessel or item to be sealed."

http://www.freepatentsonline.com/5770318.html


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Note added at 56 mins (2009-03-25 18:13:34 GMT)
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:-)

I can imagine that polypropylene has a strong elasticity, allowing it to be pierced with a syringe filled with the solution the vessel or beaker should be filled with. After filling, the syringe can be pulled back, after which the plastic film closes the hole poked into it due to its elastic properties. I cannot confirm whether this is the case with your polypropylene film, but I am quite sure I remember having seen containers stretched with plastic being pricked with a syringe when I was in my first year as a biology student.