Heat oven to 350°F. Grease bottom only of another 13x9-inch pan with shortening or cooking spray; line pan with cooking parchment paper. Make and bake cake mix as directed on box, using water, oil and eggs. Cool 10 minutes; run knife around side of pan to loosen. Place cooling rack upside down over pan; turn rack and pan over. Remove pan and paper. Cool cake completely, about 1 hour.