How can I represent a door on stage?

I'm doing Drama at AS Level and my group really needs an idea of how to symbolise a door, because my drama teacher said they're too expensive to buy. He suggested a door frame with lights around it, but I'm not too sure. The door is the main focal point in the piece as we are doing Scary Play by Judith Johnson - it's all about fears and basically people walk through a door and their fears kill them. We've also thought about split focus scenes. Ugh, sorry this is long, just stressin out!! hahahahaa