I stamped the letters on the front in blue/red to coordinate with the Montessori Moveable Alphabet. I also purchased a Moveable Alphabet Kit from The Montessori Print Shop, which I will laminate and place in each box. This kit also includes some pictures, so it will also help supplement my collection (but I like to have as many 3D objects as I can find, with a minimum of 3!).
So here is what I have in each box -