What a wonderful project to be working on to gain a deeper understanding of Harper Lee's novel To Kill a Mockingbird.
Perhaps the easiest character to create a memory box for is Dill. Given that readers are able to engage with Dill as a character, because all people have been a child, the memory box for him would easy to create.
Dill is an imaginative and devilish character, a memory box for him would be perfect to create based upon his love and enthusiasm for Dracula. The memory box can be created using imagery from both Lee's novel and Stoker's novel. Therefore, it could be started by defining a theme for each side.
Dill's love for the vampire can be illustrated by depicting the villains of literature. His own devilish personality is illuminated through his love for the horrific.
Another side of the box can depict his love for Scout. He, at one point in the text, declares that he is going to marry Scout. Therefore, one side of the box can show the relationship between Scout and Dill. This can be done by using pictures of children holding hands and images of weddings.
The third side of the box can be dedicated to the challenges he proposes to Jem. The entire reason that Jem surrenders to his fears regarding the Radley house is because of the challenge Dill puts forth. Therefore, this side could illustrate childhood dares.
The final side of the box can depict the changes which take place in Dill over the course of the novel. To begin, decide what Dill looks like to you at the beginning of the novel and show how he changes, in your eyes, over the course of the action. One creative way to do this is to decide if Dill is a static character (meaning he stays the same) or a dynamic character (which means he changes). To depict either one of these ideas, decide how to depict a static character (say using static guard or a dryer sheet) or a dynamic character (using dynamite to depict an explosion of change).