When mine were little, I used to put paper towels under the feeder...that way when they spilled the feed, it was on the paper towel and they could see it there and eat it.
By the time they were 3 weeks old, we had switched them to a bigger feeder and that helped a lot. Now that feeder is hanging several inches off the floor in the coop and they don't spill very much.
My new chicks were not eating the larger pieces of the chick grow and I used a blender to grind it up smaller and now there is very little waste. I feel like they are eating so much better now.
I use empty butter tubs for feeders. I put rocks and sand in the bottom for grit and weight. Then cut a round hole in the lid leaving a border around the hole. Now when the try to scratch or toss food the border catches it. Cut the waste down tremendously.
I second the grinding up of the Start N Grow... I find the chicks eat more, grow faster and waste almost NONE if I put it dry in a blender for just long enough to make it all small bits.
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?