Turbo Trainer Advice

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.