Recommendations for distribution packaging

If you want to package the software for distribution, a few adjustments/patches are recommended:

If you install default configuration files, you might want to adjust [NFTABLES] exe = ... to the absolute path of your nft binary. See the configuration documentation for more details.
You probably want to install the systemd units under /lib/systemd/system/ or similar, instead of /etc.
The build environment variable LETMEIN_CONF_PREFIX sets the prefix for the configuration files. In a plain cargo build this will default to / meaning that /etc/letmein{,d}.conf will be the place for the configuration files. If you use the build.sh script, the prefix will be set to /opt/letmein instead
Adjust the install prefix from /opt to something else that makes more sense for your distribution.