How to change aspect ratio of slideshow in Aperture?

I am trying to make a slideshow that has a square aspect ratio so it can fill up a square projector screen. Aperture will only let me output 16:9 HDTV ratio. Does anyone know how I can get around this? I would like to design the slideshow with a square or 4:3 aspect ratio to begin with because it will affect how the ken burns effect is used on the photos. Thanks