After some searching, I realized that they were very hard to come by. So I purchased a 45 degree erect-image diagonal and used some old telescope eyepieces and never got it to work: it wouldn't focus.
Then, just before an upcoming trip to Yellowstone, I realized the problem. I needed a Barlow lens in front to shift the focal point back far enough to accommodate the extra length.
Thus, my original project was reborn.
Here is a picture of the completed spotting scope.
Step 1: Gather the stuff (spend the cash)
1) Silicone glue (or any sturdy glue or epoxy). I used silicone because it can be undone to some degree.
2) A telescope eyepiece with a 1.25" barrel (the most common standard). I use a 25mm Celestron Plossl for wide views, and a 10mm Celestron Plossl for high power. (~$50 each)
3) A 45 deg erect image diagonal with a 1.25" barrel (for a spotting scope) OR a 90 degree diagonal with a 1.25" barrel (for a telescope). The barrel needs to be threaded on the inside. Diagonal barrels are often threaded inside to accept filters. I bought my 45deg one from Orion (~$45).
4) A 1.25" 2x Barlow. The lens element holder needs to be unscrewable from the barrel, and the threads need to match the diagonal. Mine did. It is a bottom-of-the-line Celestron ($40)
5) A telephoto lens. There needs to be room to shove the 1.25" barrel of the diagonal about 1" into the backside of the lens. (Some zoom lenses have lens elements right near the opening, which is true of my Canon 85mm to 200mm lens and you'll have to figure out how to get that to work). I'm using a Canon 100-400mm IS zoom lens. (Lots of $$)
6) A lens cap for the back end of the zoom lens (the side that connects to the camera).
7) A 1.25" hole saw. (~$8)