My adapter is not an official Canon one - it's from a company in America called Lensmate, as indicated in my
thread explaining the set up. Their address is
http://www.lensmateonline.com/ There is also a photo of the remote shutter release in that thread. I find this very handy.
I bought the Lensmate thread adapter so that the step down distance to my scope eyepiece would not be as much as from the Canon adapter with its 58mm thread. I had also heard that it was better quality.
My setup feels very secure with the piece of plumbing tubing slipped over my eyepiece, as there is a rubber ring just inside that grips nicely - no screws needed. It is very quick to get on and off and allows for the lens to zoom fully out and not hit the scope eyepiece.
The suggestion to slip the eyepiece into the thread adapter would be very risky, in my opinion. For one thing there would be a lot of vignetting - I get a little at full zoom, which is easily cropped out. If you had to pull back the zoom to get the eyepiece inside the adapter, the vignetting would be unacceptable.
Also, if you were to zoom without thinking, the camera lens would end up hitting the eyepiece glass, which could cause scratches.
The whole setup would also be very insecure and you would have to hold it in place, thereby introducing camera shake.