I went down this route last year.
Attaching my phone to my spotting scope has transformed my zeroing/practice sessions, not so much the convenience of not having to peer through the spotter, but if you are on your own, by recording video with the phone through the spotter you can then play back afterwards and spot your misses - something I struggle with through the rifle scope with the bigger calibers.
I bought a Hawke adapter which is all spring loaded so fits any phone and any type of spotter. It is a bit fiddly to get everything lined up (takes about 1 min), but then again with my spotter the eye relief changes as you zoom in, so you need to move it again anyway, so do wonder how the more expensive magnetic clip-on type cope with that. All in all I’m pleased with Hawk adapter.
A few tips based on my experience:
1. Using the zoom on the phone can give better picture quality than zooming in on the spotter. I have found a mixture of some zoom on the phone and some zoom on the spotter works best.
2. The spotter/phone combo needs to be mounted on a tripod that isn’t stood on the same table you’re shooting off otherwise the vibration of the shots upsets the picture which kinda defeats the purpose. Even a bit of wind will cause a surprising amount of camera wobble.
3. Getting the spotter/phone pointing at target does require a lot of patience!
Some photos of the Hawke adapter below. I’d hoped to attach a short video clip of me missing as well, but the technology has defeated me (again) or perhaps pride got in the way.
Hope this helps