I found a tutorial for these over at scs. Search "milk carton." So easy! All you need is a trimmer, scissor, and bone folder/scoring blade, and 7x11 heavy cardstock.
I stamped my boxes with a snowflake stamp, archival ink, before assembling. The pp on the red box is DCWV and the blue box is Eskimo Kisses. I use binder clips to close the top of the box and also attached to/from tags.