That would be a horrible solution. Besides being ugly, the space it would require would either result in a bump or thicker lid. As for a secondary display, an iPhone would be a poor choice given the size and viewing distance when mounted on a Mac.
I use a Joby pro mount and tripod. Not cheap but it’s well made and allows portrait or landscape. Coupled with EpocCam it is a very useful solution if you need a separate webcam or higher resolution.
I also added an LED light source, that attaches to the top of the mount on a hot shoe, to better illuminate my face during web calls.