There are a few ways to achieve this and my attempts were most successful using a slower lens (ie, one with a smaller object element). Fashion a crude adapter to fit over the end of the lens where the hood normally fixes and reduces the diameter to match up with the spotting scope eyepiece. Initially, you'll need some flexibility on the distance between the eyepiece and the lens but the dimensions can be fixed once you've made a few test shots.
If you "google" canon & digiscoping then you'll find a host of sites with details of setups and examples of resultant shots.
Bob