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.