Guess who asked me to sew something for him. And he practically designed it, too.
I still need to figure out a general way of calculating the size, to be able to sew without a phone.
(I didn't really measure the first one and came out rather narrow, but it fits my work phone perfectly).
Anyway, the idea is that you pull the strap and the phone slides out (magic, isn't it?)
And to do it, you attach the strap inside the cozy on one side, sew a loop on the other side (also inside) and thread the strap through the loop. This prevents the strap from sliding out of the cozy completely while allowing it to lift the phone out of the cozy.
The cozies are already in use and work quite nicely, although a wider strap (and a bigger piece of velcro) for the wider cozy would work even better.