We at robur developed opam-mirror in the last month and run a public opam mirror at https://opam.robur.coop (updated hourly).
This was nice collaborative work with @reynir and @dinosaure, and we have an article Mirroring the opam repository and all tarballs and the source code robur/opam-mirror: A unikernel that acts as opam repository including archive mirror. - opam-mirror - Gitea: Git with a cup of tea
Enjoy reading, feedback welcome.