This is a great and creative question. Dill Harris's summers were spent with the Finches, and when he runs away from home, he also winds up in Maycomb County with Atticus and family. Perhaps some items that could be included in a Dill Harris memory box could include his train ticket stubs to Maycomb, a lock of Scout's hair, some tiny trinket (porcelain figurine?) belonging to Aunt Rachel, sketches of Boo Radley, and maybe a few of the items that showed up in the knot hole of the Radley tree when Dill was actually present. Any of these items would serve to represent Dill well, and would serve as reminders of his time spent in Maycomb.