To create such a box, I'd look at the major things that represent him. Pennies, a watch and chain, soap dolls are what he gave to the kids, so they'd go in there. He kept Scout from getting knifed, so probably a knife. Maybe the scissors he was supposed to have jammed into his father's leg (with a little fake blood on them).
As for the outside, something that represents the outside of Boo. This could be gate the kids tried to tease one another past, or the tree in which Boo put things.
If there was to be a memory box for Boo Radley, I would put all sorts of things. First of all, I would put the scissors(with fake blood), with which he stabbed his father on his leg. Then I would put the newspaper he was cutting into tiny little bits. I would also put the packs of gum, the yarn, and the handheld watch. If you wanted to, you could also put the Indian Head coins which Jem and Scout found in the tree along with the watch, the yarn, and the packs of gum.