Presumably only the collision area needs to be underground to minimize noise. The rest of the ring can be above the surface. Building things on the moon is hard

Presumably only the collision area needs to be underground to minimize noise. The rest of the ring can be above the surface. Building things on the moon is hard especially materials much easier to ship them from Earth using Starships or a derivative. Can start with a small ring made from a few ships and build a bigger one and finally a great circle one. Actually only need 500 or so towers 100 meters high each supporting a magnet. Starship can deliver 100 metric tons to LEO so could deliver a significant fraction of that to the lunar surface. Maybe 25 Starships performing 25 lunar missions each would be enough assuming LEO refueling and payload transfer.