One of the issues is that rectangle-packer, while it exists on PyPi, is not part of Nixpkgs, and I’m not sure what would be the idiomatic way to include this in my expression. Would it involve the use of the fetchPypi function?
I just recently packaged a Python app where I had to also package some dependencies, you can use this as reference: https://github.com/NixOS/nixpkgs/pull/220305/commits/e2b1e574da314e4aaf771f4dd9f522ac2b37bf56
I’ve done this before in some other PR and ultimately was asked by a reviewer to turn these dependencies into top-level packages. 
Another issue is that the setup file specifies both pyQt6 and pyQt6-Qt6 as dependencies. These packages both exist on PyPi, but the latter does not seem to be in Nixpkgs. Is it perhaps contained within the first one?
I don’t know tbh, but I’d say try building it with pyqt6 and see what happens 
The last problem is the exif dependency, which seems to refer to this program (an outdated version of which is in Nixpkgs).
The current version of the Python exif package in nixpkgs is 1.3.5.
I see beeref doesn’t specify a version for exif so 1.3.5 might work. Otherwise you’ll probably have to upgrade exif in nixpkgs separately first.