I just checked Ballistic products... they do have 20 gauge Dangerous Game Slugs.... $14 bucks for 25 slugs. I am sure they can give you a powder and shell recommendation to go with that slug.

Or you can stay with Round Balls in a slighly smaller diamter than your bore using a shot cup to hold them in.

Plus, I shoot slugs from my double smoothie sometimes and have used factory loaded slugs with acceptable point of aim results... I go online and buy 10-15 slugs of each flavour and do a 25-30 yard test. The slug that gets the best Point Of Aim results from BOTH barrels gets the next order for more.