This is probably one of the best advent calendars I've seen. My step mom made each of my kids one of these. Each year she buys all the goodies to go in them and I put it together. Even though my son is 13 he still loves opening one of these boxes each day.
Each box is 4X4 and you will need 25 for each calendar. Only the lid is stamped. The backing is a piece of foam core board (20X30) painted black and then Christmas-y type paper used as the background on top. Each of the squares has a letter in it that spells Merry Christmas and then the name of the child.
The boxes were the hardest to attach (and make them stay that way!). I know this because my step mom used hot glue which had come off by the time we got these home. I wanted something strong enough to hold them so we could hang these on the wall.
For my daughters, I punched a hole through the box bottom and foam core and put a brad through to hold the boxes. For my sons I used Gorilla Glue and that seems to be holding well.
On each box I punch hold in the bottom on opposing sides. After I put the goody in, I ran ribbon through the holes, placed the top on and tied the ribbon to hold the box lid on nice and tight and keep prying eyes out!!