YOu need a very fine substance that will work its way down through the shot you use to fill in all the gaps between the shot. Malt-O-Meal may be fine enough. Corn meal is not, unless you buy one of those package corn meal mixes, like Jiffy . That stuff is ground as fine as flour, and should work. However, most duck and geese shooting occurs on rainy or overcast days. The fine natural materials will soak up water out of the air like a sponge. That is when you curse the stuff, and wish you had brought some PufLon instead. The synthetics( grex too, of you can cannibalize some modern shells to get it ) won't absorb water, either in your pocket, or in the gun. They will blow away in the wind because they are so light, so protect the muzzle of the gun with your back before you pour any of the stuff on your shot. You have to pour, then tap the side of the barrel to vibrate it down through the shot load, then pour some more, then tap that down, then pour more, etc. It is a slow operation in a ML shotgun, but you can use the stuff. You might want to work out how much of the stuff will fit in your particular load of shot, and then "pre-measure " that amount to carry to the field in a separate container. Then you will know when you have all the filler in your load that it can hold, and be assured that the filler has filled ALL the gaps.
When reloading shotgun shells( smokeless or BP) you can use a case vibrator, used to settle large powder charges in small bottle neck cases, to settle the filler in the case. You don't have that with a ML, of course, so it takes much more time to do.
If you mix the filler and the shot together, they will separate when you pour the shot into the barrel, as the lead shot will be so much heavier. But, some of the filler will go down the barrel with the shot, and that is a beginning. I would try this last technique before I stood around in a goose pit trying to jiggle filler down into my shot load when Canada Geese are coming into the set up. In fact, with any waterfowling, you might want to have a second gun ready to cap and fire, if regulations permit it.