A Camera Obscura is nothing more than a telescope that has it's prime focus a screen for viewing. They are normally made in a building so that the entire room in which the telescope light path is is in the dark. The Objective is usually placed so that the light path is vertical and a mirror is used to direct the light into the lens (normally what is used). A horizontal path is usually the other method of usage. This allows the access to all of the sky above a certain angle. The actual telescope design may be of any design that you want. Most of the designs are of a long focal length as most of them are for Sun Observation. That's pretty much the long and short of it. Have fun.