posterboard just like wood has a grain. it will roll perfect in one direction and crease in the other. always make sure you check and make sure the grain allows your part to do what you want before cutting out. if it want's to crease then turn it 90 degrees and try again, bet it will work fine.
2L bottle will be cool though. not sure that i have seen anyone make the turtledeck out on one before. lot's of canopies but never a turtledeck.
good luck,
me