I found that 28 balls layed in four layers of seven balls works great. The total weight comes out just a tiny bit more than a single cannon ball. In my case the cannon ball weighs 20 ounces and 28 pieces of grape shot weighs 21 ounces, close enough.
As with the powder charge, I fashion an aluminium foil cylinder, drop in each layer of balls and twist the top shut. This gives you a sort of bastard load, it ain't true grape and it ain't true cannister.
You will probably want a sabot between the powder and shot. I use an adjustable hole saw and cut them out of scrap plywood. The sabot should be as tall as the bore diameter. In your case about 44mm tall give or take, it's not real critical. Primarily the sabot keeps the expanding gas from blowing through the shot load.
If you should opt for smaller size shot, the method is pretty much the same. Pour in shot about equal to the weight of a round cannon ball.
Some fun targets can be made from scrap sheet rock/drywall. I cut it to about 2' by 6' and stand several pieces out at various ranges to simulate troops in the open.
I use the same powder charge as with a single cannon ball.
Have fun.
Bill