This is a mechanism I designed and built to hold my wife’s phone on her motorcycle.
The phone fits, in landscape orientation, into the black clip to the left of the image. This allows good visibility of the instruments as well as the phone screen. It also enables the phone to be quickly taken in and put back again. The mount was only a couple of pounds in materials, compared to the £100 or so a commercial system would cost. Thus far, it seems rigid enough and does not vibrate very much at all (the bike has a single-cylinder).
I ended up using the only Meccano I can get these days (the horrible new type, made in China from old tin cans).