I too like the pizza boxes for WIP. I bring them home and put contact plastic on them, to make them all pretty! Have an assortment of sizes, to fit the projects!
Makes it nicer when I'm taking the pieces to LQS or for a Show and Tell!
Add an extra long shoe lace threaded thru the box top and bottom and you have a nice way to tie the boxes shut safely. PLus you can slide your squares in under the thread to hold them in place.