In contrast to many of the excellent ideas here, I’ve been using a low tech solution. The tented centre bar isn’t sufficiently load bearing to use on its own. On the other hand, the octo feet are solid but quite bulky.
So I stuck a cardboard box to the bottom of the tented bar. If it is exactly 2cm thick it takes all the pressure off the connection. And then you can use any flat surface, such as a beanbag breakfast tray.
It’s not quite armrest mounted, but it comes out about the same position.