An external camera viewfinder with zoom cabability

Specify the optical elements required and their grouping to allow a manual zoom external camera viewfinder. It should cover a 35mm film camera's angle of view for a 28mm to 90mm lens. Ideally it should allow for a minimum of of .85x magnification but a 1x magnification would be ideal. The glass elements should be easily available from an online store like Edmund's etc.. I am willing to do away with the zoom function if its easier to set the viewfinder to specific focal length coverage for 28mm, 35mm, 50mm and 90mm.