The little compartments are ideal for storing buttons, brads, paperclips, what have you.
Here are your free files, pdf for hand-cut, or Silhouette for digi-cut:
Milk Carton Printer's Tray
Step-by-step how-to photos follow. I used 160gsm photocopier card. The smaller size is 80% of the size given. You may have to tweak the size of the compartments by a degree or so to get a good fit, depending on your paper choice.