Sliding Projector ceiling mount?

My projector is currently living on a shelf in my den in a very ad-hoc manner. I want to ceiling mount it but I have an issue. My projector doesn't have much zoom, but I like to move it further from / closer to the screen to vary the image size - I move my seating arrangements around a lot based on how many people I have over and sometimes I want a smaller 'screen size.'

Is there any sort of ceiling mount that would allow my projector to slide along some sort of track so I can alter the image size? Or, alternatively, is there any sort of after-market zoom lens I can use?

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