Okay, so re-posting my method. I had it written out last time, and there wasn't many things wrong with it. I'll just touch it up according to how I did it. Also, my first experiment was alright. I'm not sure if I did it correctly with the correct timing/procedure/placement of containers etc. etc. So I'm going to do it again over the next week or so, and then maybe a third time round? I also took a few photos, which will probably be up soon.
Anyway, here's the method:

1. Pour 250mL of water into container
2. Cut out a 30cm x 30cm sheet of plastic.
3. Place plastic over the container and securely wrap it around all edges
4. Clearly label your container
5. Wait 4 days
6. Take off sheet of plastic and measure the amount of water left in container
7. Record observations
8. Repeat steps 1-6 with the different brands of plastics