Man, hate to say it but the bender isn't really the problem, it's what you're feeding it. Those craft store ones are made for scrapbook paper and thin cardstock, so a cereal box is honestly pushing it. Real corrugated needs a proper scoring tool or a steel rule die, not a butter knife either since that just crushes the flutes and makes it weaker. If you're doing castle walls, try scoring both sides with a dull craft knife along a straight edge, then fold. Way cheaper than $75 and your glue gun gets a break.