Sorry for the multitude of replies. I am replying while I am catching up with the huge amount of info on CloudyNights and I should have refrained from replying until I read it all. Having said that...
I ended up using threaded adapters to mount my ASI6200MM and filter wheel on the Esprit 80ED to avoid light leakage. ZWO provide this "how
to achieve the right back focus" chart and suggest using the screws but that wasn't good enough for me. I want a light tight solution and I found it though I had to buy a few additional rings to achieve the correct back focus experimentally.
Unfortunately I am away from home so I cannot look up how I did it but it involves using the M42-M48 ring to attach the camera to the filter wheel and both the 16.5 mm ring to attach the filter wheel to the flattener of the telescope. HTH.