I've actually just modified the strap, bought a longer set of straps and added a second Velcro dot on the inside of the pillar. I had a few high speed openings of the windows as the older straps frayed. Now they're double secure.
I've actually used those magnet phone holders before. Worked fine for a phone, but I'd have reservations that they have the strength to hold the windows closed at 52mph with a bit of a side wind.