Hey Tinker, Alan - lots of guys here use their 20 bore flintlocks for hunting moose. They do as good a job to 80 yards or so, as any gun and better than most modern stuff.

You've experience with 12 bore and blals - I'd just duplicate the types of wads you are familiar with and go from there.
There are moulds available from .590, .595, .600, .610 and .615, the mominal 20 'bore' size. Rapine makes several in these sizes, I believe. Most comon for the smoothbored flinters is the .595".