I was thinking of making a shoebox cake and gumpaste shoe for my aunt's birthday but was unsure about the box top. Do most people use non-edible materials (like styrofoam) for the top or is cake the norm?

I would like to know as well. I was thinking of making one for my dauther's bday.

You can use gum paste or fondant with tylose added. Sugarshack has a wonderful Boxes & Bows DVD that shows different ways to make them www.sugaredproductions.com

I used cake for mine but I think most people don't. It did almost break in half and it was a bit difficult to cover the whole thing (the underside doesn't crust)
Another reason I think people don't use cake is because the cake can sweat and go mouldy so if you so use cake don't make it too far in advance.
(i am not an expert though so wait til you get more advice!)