when I've done this I just throw all kinds of stuff up under and around the trees...scraps of fencing, old shingles, scrap metal, branches and sticks, anything that they can't get through easily to dig down. Once the trees are 2 or 3 inches across at the base, a cluster of rocks,
concrete chunks, bricks, etc. right around the base of the trunk is usually sufficient. The benefit of running chickens continuously under fruit trees can be dramatic. In southern Georgia years ago I was able to get 75% worm free fruit on early peaches this way. Even the people from the extension office came out to see it!