I like to use the plastic zipper storage bags as. Depends on the project size but most often I use the one gallon and two gallon size.
Another idea: One of the ladies in our class goes to the local pizza place and asks them if they will give or sell new pizza boxes. She says that most often they just give her one or two at a time. I don't think she has asked for more than two at a time tho.
She says the medium size is perfect for 12" blocks. Easy to transport to class too.
DeeDee