So there's a good number of examples of how one can mirror hackage, but less on how one could mirror stackage.
If one already had a mirror of stackage, and the mirror's users were content to use cabal, a minimal mirroring could be just to grab all the cabal.config files from the snapshots
(which one can discover via https://www.stackage.org/snapshots
If the mirror's users wanted to use stack, though, I'm not clear on how to configure stack to use a local mirror, or what would need to be mirrored to be sure that all of stack's features (templates, installing ghc, etc) work.
Noah Luck Easterly