I’ve made hakyll-flakes, a flake that provides library functions for building your own hakyll website using flakes.
The main feature is just that it pins a version of nixpkgs / hakyll that will actually build (I’ve often had hakyll fail to build due to dependency version mismatches).
It also produces a nice set of flake outputs, including a
website package that builds your site from start to finish,
builder that just compiles your hakyll source file, and a
defaultApp for convenience.