I actually made something very similar a few years back and now wish I’d had the courage to follow it through!
It’s an ingenious piece of gear for your bike and works really well. Used it for the first time whilst riding indoors and found it extremely useful rather than my own botch job system. My only disappointment and error is that I thought my current protective case was magnetic and would attach straight to Maggie but I clearly didn’t fully understand its compatibility requirements and therefore have to remove my Iphone from its case each time which tbh is no great hardship or effort. On the plus side I have noticed the rear of my Iphone was badly cracked and therefore the magnetic properties are much weaker and not as strong as a fully functioning one. I will only be using Maggie on my indoor turbo trainer so it’s unlikely to ever fall off anyways.
Well worth the money imo.
The only constructive criticism is that the magnet could be stronger - I’ve managed to knock my phone off a couple of times, mid even on Zwift, just when you don’t want to stop and get off to pick it up. Otherwise great for the money!
Handy when doing Zwift or TrainerRoad workouts, has a good hold of my phone so it doesn't slip off during workouts.
Bit expensive but I guess if it was too expensive, for me, then I'd not have bought it.