A little bit of self-adhesive hook and loop tape (Velcro) works for me.

A guitarist I know locks his phone in portrait-mode and (while playing seated) wraps a home-brew Velcro strap around his right leg just above his knee and sticks his phone to it. Easy to see and adjust, removes instantly. Stable when he taps his foot.

I can see it might be a problem if the display auto-rotated as he moved.
)BOB