I think 20W is your only option as MagSafe is USB-C, and as far as I know, the lowest Apple USB-C power brick is 20W.
I will say this. If you charge your AirPods with a MagSafe charger, there’s a very good chance the case is gonna get hot as hell. That’s what happened to me. I recommend 5W lightning. Others will say I’m nuts, but I just don’t want to fry my stuff.