I was searching for the same thing and found your post. It’s late, but what I ended up doing was to glue one of those MagSafe dashboard mounts into the case of my old phone (which had one of those Garmin stickers on the back) and then cut the rest of the case away.
To position the MagSafe mount, I stuck it to the back of my phone and then (as this was a clear case) just put a few dots on the back.
Works well enough for TrainerRoad on my rollers, probably wouldn’t take this on the road though. But then there’s a reason the bike has a Garmin mount. 
The mount I used: https://www.amazon.com/gp/product/B093Q26FLT/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
Result:
