It seems to be the billing, the side-to-side head motion that causes the food to be spilled & wasted. I wish those smaller 3# plastic feeders had the slats like the larger 11# feeders do, that seems to cut down on the billing & the waste.
Perhaps you could cut a piece of hardware cloth to fit the pan of your feeder -- just be sure there aren't any sharp edges to cut little faces. I fit pieces of .5 X .25" hardware cloth into those metal rabbit-cage type feeders to cut down on the waste.
I have older chicks, 8-10 weeks old, in a tractor. If they spill too much feed on the ground I remove the feeder until they've pecked up the stuff on the floor.
The worst feed wasters are the Mama hens, they love to scratch vigorously at the feeders to get food for their chickies and toss a lot of feed around.
I think I've seen home-made feeders that might reduce the waste, wide PVC pieces with large holes drilled in them, and attatched to a heavy base so it doesn't get tipped over by the older stronger birds. Anyone have a picture of one?