Ive built a few for folks with loaders and the thickest Ive used was 1/4 and 5/16. I made for an old 8N Ford had 1/8th sheet metal for the shell and 1.5 by 3/16 flats for the bottom ribs. I also used the 1.5x3/16ths to make the cutting edge later I used to for a support for when I added a bucket extension to it. The very last bucket I made was 3/16ths sheet I ordered sheared to the exact bucket width. I took it over to my cousins fab shop and used his homemade Brake to form the bucket shell then I made the side plates, brackets, and ribs.