I can't really visualize Scout without her beloved overalls (and, if you've seen the film version, the "wife-beater" t-shirt that she wears underneath). I would probably decorate the outside with cutout denim overalls, with the overalls pocket on the top of the box: That way, other remembrances can also fit into the pocket. The overalls straps could also be included and used to secure the box. A photo or picture of Dill seems like a fitting addition to the outside of the box so she can see her "permanent fiance" without having to open it.