Over 20 years ago we kept an RT-300 (rotary tray instead of straight try, and body by Singer of Europe) stacked with an Ektagraphic and stock lens. These were in a darker section of the sales floor and pointed at a screen in an alcove. All we had to do was to project a pair of in-camera dupes on Kodachrome 25 (before they took all the silver out) and anyone who could afford it went for the Colourplan. BTW, the RT-300 (and perhaps others of the Pradovits) bounced the light off a passband mirror that didn't reflect UV or IR, so you got little to no damaging UV or IR light passing through your projected slide.
You can get better lenses for an Ektagraphic than stock, both from Kodak and other companies like Buhl, but they'll cost you. We use a Buhl lens with shift in an Ektagraphic where I work, to avoid keystoning. Thing I hate about the Ektagraphics is that they often go the wrong direction when you hit the remote button, mostly on the first attempt to switch directions. Anyone know what causes this?