Do you need to look at the phone?
When I ride outside I just stick mp3/phone in pocket or clip it on a sleeve as it's only for tunes.
No window sill nearby you can rest it on?
I'd still get a quadlock unless you're really sure you're never going to ride with a phone on the bike in which case that bone collection thing is pretty quick to fit/remove.
I posted links to quarter turn phone mounts in another thread recently if you want to do your idea with the case onto Garmin mount.
The whole bone mount can be taken off so might be cleaner than a Garmin out-front mount. Same if you're using the standard mounts. Bone might be advantageous if you plan on changing phones any time soon - it fits different sizes whereas your custom case will probably not.
Do you need to look at the phone?
When I ride outside I just stick mp3/phone in pocket or clip it on a sleeve as it's only for tunes.
No window sill nearby you can rest it on?
I'd still get a quadlock unless you're really sure you're never going to ride with a phone on the bike in which case that bone collection thing is pretty quick to fit/remove.
I posted links to quarter turn phone mounts in another thread recently if you want to do your idea with the case onto Garmin mount.