Thick Phone books and Big old books.
One thing that i have used for a Long time is Old Phone books, or since i get HUGE catalogs at work like the big Grainger or HD supply books i use the old ones i get. Tape them up real good and use them, If after a few shots they are not totally destroyed I'll tape them up again and use some more. Sometimes I get to dig out some spent bullets to see how they did. Everyone else can keep their Steel targets.