Not a purchase per se, but I whipped up a simple phone tray to stick my phone in on the trainer, in Fusion 360 and 3D-printed it. With an adhesive Garmin style twist mount on the bottom it sits neatly in the computer bracket and is quick and easy to grab either the phone from it, or also replace it with a computer for use outdoors.
The adhesive twist mount:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1