Install opam monorepo.
It doesn't install yet on 5.0.0, but you can install it on a different switch and reuse the binary.
Ensure you have the alpha and duniverse repositories configured:
$ opam remote
[NOTE] These are the repositories in use by the current switch. Use '--all' to see all configured repositories.
<><> Repository configuration for switch 5.0.0~alpha1 <><><><><><><><><><><><><>
1 dune-universe git+https://github.com/dune-universe/opam-overlays.git
2 alpha git+https://github.com/kit-ty-kate/opam-alpha-repository.git
3 default https://opam.ocaml.org
opam monorepo lock
opam monorepo pull
git checkout 754836818dc5c -- duniverse/eio duniverse/ocaml-cohttp duniverse/luv duniverse/httpaf
dune build ./example