The best things to present in a shadow box which represents Gary Paulsen's Hatchet would illustrate (or highlight) the things important in the novel. These objects can represent literal objects or ideas in the novel, or they can represent the idea of the object (what the object symbolizes).
Most importantly is Brian's hatchet. It symbolizes the one thing he carries from his "old life" to his "new life." The crashed air plane also represents Brian's transformation (given he would not has changed without the crash testing his ability to survive). One could also place a pair of lips in the shadow box (which symbolizes the Secret Brian possesses about his mother's lover). Fire is another important symbol in the novel. By using the hatchet, Brian is able to start a fire. This speaks to his ability to learn and survive in the wild.
One last thing which could also be included is representations of both the urban world (New York) and the wilderness. This symbolizes the distinct places the novel depicts. It also shows the problems which can arise when one is used to one way of life and is thrown into another.