Re: Building 15x15 camera
Gordon's post echoes my initial reaction to your question...nesting boxes would be the cheapest possibility. You may have to deal with the possibility of having to load one sheet of film into the box in a darkroom and then getting only the one shot but pinhole photogs have been living with this technique for years.
Sounds like a great project.
It's only a deal if it's where you want to go. Find your travel deal here.