Trying to implement this... https://github.com/gazoscalvertos/Hass-Custom-Alarm/blob/master/guidance/configuration.md

custom-alarm.nix

{ stdenv, fetchFromGitHub }:
with stdenv.lib;

stdenv.mkDerivation rec {
  name = "hass-custom-alarm-${version}";
  version = "1.3.1";

  src = fetchFromGitHub {
    owner = "gazoscalvertos";
    repo = "Hass-Custom-Alarm";
    rev = "v${version}";
    sha256 = "1xlwsyg45kmvx3gvkv7k5imbhkxan0s573lalkcqcflf078w7555";
  };

  dontInstall = true;
  unpackPhase = ''
    mkdir $out
    cp -r * $out/.
  '';

}

hass-custom-alarm.nix

{ config, lib, pkgs, ... }:

with lib;

let
  cfg = config.services.hass-custom-alarm;
  hassConfig = config.services.home-assistant;
  configDir = hassConfig.configDir;
in {
  options.services.hass-custom-alarm = {
    enable = mkEnableOption "Home Assistant Custom Alarm Panel";
  };

  config = mkIf cfg.enable {
    environment.systemPackages = with pkgs; [
      hass-custom-alarm
    ];

    systemd.services.hass-custom-alarm = {
      description = "Home Assistant Custom Alarm Panel";
      before = [ "home-assistant.service" ];
      preStart = ''
        mkdir -p ${configDir}/custom-alarm
        mkdir -p ${configDir}/www/alarm
        mkdir -p ${configDir}/www/lib
        mkdir -p ${configDir}/panels
        mkdir -p ${configDir}/custom_components/alarm_control_panel

        ln -s ${pkgs.hass-custom-alarm}/alarm.yaml ${configDir}/custom-alarm/alarm.yaml
        ln -s ${pkgs.hass-custom-alarm}/www/alarm/alarm.css ${configDir}/www/alarm/alarm.css
        ln -s ${pkgs.hass-custom-alarm}/www/lib/countdown360.js ${configDir}/www/lib/countdown360.js
        ln -s ${pkgs.hass-custom-alarm}/www/lib/jquery-3.2.1.min.js ${configDir}/www/lib/jquery-3.2.1.min.js
        ln -s ${pkgs.hass-custom-alarm}/panels/alarm.html ${configDir}/panels/alarm.html
      '';
      serviceConfig = {
        Type = "oneshot";
        RemainAfterExit = true;
        User = "hass";
        Group = "hass";
        Restart = "no";
        ProtectSystem = "strict";
        ReadWritePaths = "${configDir}";
      };
    };
  };

}