mirrors.nix

{
  nixpkgs.overlays = singleton (self: super: {
    mirrorsPath = self.path + /pkgs/build-support/fetchurl/mirrors.nix;

    customMirrors = import mirrorsPath // {
      hashedMirrors = [
        http://tarballs.nixos.org
        http://example.com
      ];
    };
    
    customImport = scopedImport {
      import = path: if path == mirrorsPath then customMirrors else customImport path;
    };

    fetchurl = (customImport self.path { }).fetchurl;
  });
}