Little confused. You say you need to make a passageway out of cardboard a person can walk through, but you're out of ideas. What's the idea you need for the passageway?
Regardless, when I'm out of ideas, I'll get away from the computer environment go rest or something and let my mind free. I also like to sketch my ideas on a whiteboard or sketchbook. Maybe take a walk. Who knows, something along your path might give you an idea.