You can also use rockwool cubes or Rapid Rooter plugs cut to fit around the stem, wrapped in plastic wrap with the top part open. Works very well, imo.
There are tons of ways to do air layering. I’ve been thinking of buying some purpose-built cups for it, actually. Don’t have the link handy because I’m mobile atm.
Air layering is great for people like me who need to stick to strict low plant counts, not sure there’s a lot of benefit otherwise.