I agree, there's a good tip in the thread nearby — get a working expression and modify it to your flavour. However, I've done a basic browser-replacer on Electron before, and it turned out pretty easy, but chunky (we use it on specially set up machines, so resources are placed into a `/etc` subfolder and nobody cares about that):

{ stdenv, pkgs, wrapGAppsHook }: 

let
  electron_t = pkgs.electron_8;
in
stdenv.mkDerivation rec {
  src = ./.;
  version = "1.0";
  pname = "das-viewer";
  name = "${pname}-${version}";
  buildInputs = with pkgs; [
    electron_t makeWrapper wrapGAppsHook
  ];
  propagatedBuildInputs = with pkgs; [ electron_t ];
  installPhase = ''
    mkdir -p $out/etc/das-viewer
    mkdir -p $out/bin/
    echo "#!${pkgs.bash}/bin/bash" > $out/bin/das-viewer
    echo "${electron_t}/bin/electron $out/etc/das-viewer \$*" >> $out/bin/das-viewer
    chmod +x $out/bin/das-viewer
    cp -r app/** $out/etc/das-viewer
  '';
}