I agree with both Mayayana and Clare Snyder here. Plastics have the largest coefficient of thermal expansion. The lowest coefficient is with ceramic materials like brick, glass, concrete, mortar at 5 to 12 parts per million per degree Celsius. Then metals at from about 12 to 40 parts per million per degree C. And finally plastics, which for PVC is about 200 ppm/deg. C. if memory serves.

But, you should keep in mind that the 3 mm is for the largest dimension of the sheet of plastic. (4'X8'?) If you're only using 1/4 of a sheet, you may only need 1/4 of that 3 mm to allow for thermal expansion and contraction. But, since you're using rivets which won't allow for any sliding of the plastic as it expands or contracts, you need to allow some way for the plastic to move or it will buckle when it expands. You may want to use nylon screws and nuts instead of rivets. Most places that specialize in fasteners will be able to order nylon screws, nuts and washers for you. That way you can fasten the plastic down, but still have some slippage of the plastic between the nylon washers to allow for thermal expansion and contraction.