This is how I make "plastic" for my projects.
All you need is:
-Aluminum foil
-Styrofoam (expanded polystyrene)
-Iron
-A towel or ironing board
-Common sense and safety

WARNING: DO NOT ATTEMPT TO THIS INSTRUCTABLE. IT INVOLVED HIGH TEMPERATURES THAT COULD CAUSE SERIOUS BURNS, PROPERTY DAMAGE, OR EVEN FIRE. THIS PROJECT MAY ALSO GENERATE HARMFUL FUMES THAT COULD CAUSE PERMANENT BRAIN DAMAGE.
I AM NOT LIABLE FOR WHAT YOU DO WITH THIS INFORMATION!

I hate disclaimers (but I hear getting sued is worse).

Step 1: Preperation

Lay down a towel or ironing board on the surface you want to work on so it doesn't get hot or burn. I recommend using an old one that you don't care about because it will probably get burned.
Plug in the iron and let it get hot. The temperature doesn't matter, but if it hotter it will melt faster. I'm not sure but when it gets hotter it probably releases more fumes.
Take your Styrofoam (I use packaging from a printer and spacers for bottles) and break it into smaller pieces (about 3"x3"x2"or about 80x80x50 mm) The size doesn't matter, but it makes it easier to melt.

Step 2: Melt It!

Take a large (about 8"x20") and put a few chunks in and fold it in half so that the bottom of the iron doesn't come it contact with the foam. If it does it will ruin the iron. I would recommend using a cheap garage sale iron or a used one so that if it does melt on it it doesn't matter.
Place the foil and foam on your towel or ironing board and slowly press the iron down on to the foil. After a few seconds the foam should start melting. Keep melting it until it is only about 1/8" or about 3 mm thick. Take the iron off and let it cool. After it is cool peel back the aluminum foil. If you want bigger pieces repeat this process several times.

Step 3: Final Thoughts

It's Done!
This project is extremely easy and uses foam that most people would throw away. It can be done in as little as 5 minutes and doesn't require any fancy equipment or costly and dangerous chemicals.

I hope you enjoy doing this project as much as I did. Please feel free to comment, or suggest other alternatives. I would love to see what you make with it.

And If you like it please rate it.