Justin, Yes the sleepers are made of plastic. The red Pete's sleeper is made out of CD cases but for the bigger sleepers I bought plastic sheets from the hobby store. Here are some more pictures for you of the sleepers. Just want to remind every one, these were my first scratch builds and were done about 7 years ago, back then I didn't know all the tricks and things to modeling so the detail on these are crappy. Now that I have learned allot more tricks I am going to redo these.
Here are a few morw pictures.