The tool F-Engrave is distributed as a single script that is intended to be edited by the end user, so packaging it as an application in Nixpkgs would require the end-user to use derivation overrides to customise the script by patching, which is a more tedious workflow than simply editing the file in place. Hence this flake providing a devShell with all of F-Engrave's dependencies, including the bundled helper program ttf2xcf_stream.
To use, run nix develop github:aidalgol/f-engrave-flake. This will unpack the f-engrave.py script from the F-Engrave source zip archive to the current working directory, if the file does not already exist.