Record the measurements of the length, width and depth of the plastic. Doing so will help you determine if the plastic contracted and how much it contracted.
Make cold plastic contract even further by placing it in a freezer or outside for a few hours in frigid weather. Measure the plastic to see if it contracted.
Give small pieces of cold plastic a shot of canned air to see if that causes a contraction. Place it near a block of dry ice, in an insulated container to cool the plastic even further. Don't touch the dry ice to the plastic, but get it close. Measure the plastic to record the contraction.
Look at your notes and determine which method made the plastic contract the most. Your notes can also tell you how cold the plastic has to be to contract. If the plastic contracted in frigid temperatures, it may not be the best material for an outdoor project. Plastic the only contracted with the dry ice can withstand extreme temperatures.