Thingiverse denizen Robert Lorincz created this adjustable 3D-printed geared smartphone holder that you can adjust to find your preferred angle:
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I think it would be useful as the buck you use to experiment with different angles while working on a more finished product. Heck, I think a scaled-up version of this could also be useful as the skeleton for a chair buck.
Either way Lorincz deserves kudos; this is not bad for something “designed over Sunday.”
The files are here.
Source: core77