Although this is a knitting program, I think it would work fine cut out of fleece:
http://www.themarysue.com/chicken-sweaters/
This one shows how you can position buttons and buttonholes:
http://thechicken-wrangler.blogspot....tery-hens.html
Edit: It needs to be snug so the chicken doesn't get its foot caught in the material. The knitted sweaters have a bit of an advantage in that respect:
http://stitcheryprojects.com/2015/12...icken-sweater/