The chest

Devon was obsessed with becoming an eagle scout, and he had to do a project to get in.  He was making a wooden chest.  Then, when he died, it was left unfinished in his room.  It just lies there for most of the book, but later on Caitlin uses it to find Closure (See closure for more detail).  The chest reminds her of Devon, and she goes into Devon's room just to see it.