I used to watch a family of coyotes raising puppies in the gulch behind my house (Coyote Gulch -- makes sense) with a spotting scope. I wanted to take some pictures of the puppies, so I first tried just holding the phone lens against the eyepiece, which was not easy at all. Very difficult to brace the phone and keep it straight and centered. I took a look at a few of these types of phone mounts (and the prices) and decided to try cutting a squared notch out of the cheap lens cap that came with the scope, since replacing it would only be a buck or so. Not really much to lose if I messed it up.
My cap modification made it easier to brace the phone and I got pics that were recognizable as puppies. It doesn't work very well, but well enough for my purposes.
A mount like one of those looks like it would be really nice, but be aware that the zoom on a phone is a digital zoom. It will get noticeably more grainy as you zoom in. But more than just added distance, having a mount like this would make it easier to take pictures/videos and show a group of people what you're seeing. Let us know how it works if you do decide to pick one up!