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)
Attach the lens element to the end of the diagonal (the end that will stick into the lens).
Step 3: Drill the cap
Step 4: Make sure it can focus
Point the camera lens toward a distant object.
Put the eyepiece in the diagonal/Barlow assembly. Put the drilled cap on the lens.
Put the Barlow end into the camera lens. Hold it roughly where it will be when it's glued to the cap. Look through the eyepiece and adjust the camera len's focus. Make sure that the system can focus. If not, try moving the Barlow out a bit. For my Canon 100-400 lens, the system could focus for the longest (400mm) case when the diagonal was right against the cap. If this is not the case for you, you may need to fashion a spacer between the diagonal housing and the cap. The spacer must be the same thickness all the way around to guarantee that the barrel of the diagonal is aimed straight down the barrel of the lens.
Step 5: Glue the diagonal onto the cap
Put the diagonal into the hole in the cap. You'll want the part that holds the eyepiece to be outside, and the shiny chrome part of the diagonal on the inside.
It's important that the diagonal barrel is exactly perpendicular to the camera opening (otherwise, the image will suffer). Check the fit. On mine, there was no issue and the housing of the prism in the diagonal sat flush against the lens cap. But if it tilts, you'll need to shim it or file it till it doesn't.
Connect the cap to the lens and figure out which way you want the diagonal to be facing. For a 45 degree one, you want the angle to point away from the tripod mounting hole on the lens.
Mark that orientation. Check it. You won't be able to change it once the glue sets.
Glue the diagonal in place.
Let it dry.
Insert an eyepiece into the diagonal and you're ready to use your new scope.
Step 7: How did it perform?
It turned out to be an ideal instrument to show the kids pronghorn antelope in Lamar Valley, Yellowstone.
Pointing the thing with a generic tripod (no slow-motion controls) is pretty tricky. I tend to prefer 25mm eyepiece so the magnification is not too high, and the field of view is bigger.
I estimate the magnification to be about 32x with the 25mm and 80x with the 10mm eyepiece. I think the Barlow is doing a little more than 2x in this configuration, so it may be a bit higher. Since the clear aperture of the lens is about 65mm, I could probably go to a 6mm eyepiece and still get more out of the image, but my tripod would be extremely hard to aim at 130x.
If you make a telescope (as opposed to a spotting scope), one drawback of this scheme is that you can't rotate the diagonal. On an altitude/azimuth mount, this may not be a big deal, although when you look at things low on the horizon, you may have to lower the mount since you can't spin the diagonal sideways.
I doubt it will work well on an equatorial mount, unless you can spin the whole telephoto lens in its mount. Having said that, the Canon lens cap can actually go on in 3 different ways (120 degrees apart), so it may be an ok compromise.
I should also mention that although the lens is a zoom lens that can be as short as 100mm, it actually will not come into focus for distant objects unless the zoom is set between 200mm and 400mm.