Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 13x9-inch pan with baking spray with flour. Make and bake cake mix as directed on box for 13x9-inch pan--except use 1 cup water, the oil and eggs. Cool 10 minutes; remove from pan to cooling rack. Cool completely, about 1 hour.
From center of cake, cut one 3-inch crosswise strip (see diagram). Cut the strip diagonally in half to make two 9-inch triangular wedges. (Discard 1 cake wedge or reserve for another use.)
On tray, place a 9x5-inch cake piece. Stir food colors into frosting to make golden yellow. Spread 1 tablespoon of frosting on 1 edge of triangular wedge of cake. Attach wedge, frosting side down, to 9x5-inch cake piece on tray, placing wedge along top edge of larger cake piece. Freeze all cake pieces 1 hour.
Spread 1 tablespoon of frosting on top edge of triangular wedge of cake. Attach remaining 9x5-inch cake piece to cake wedge to look like partially opened treasure chest. To seal crumbs, frost cake with a thin layer of frosting. Refrigerate or freeze 30 to 60 minutes to set frosting. Spread remaining frosting evenly over entire cake. Pull fork through frosting to look like wood grain.
Use pull-and-peel licorice to make handles and straps. Fill chest with chocolate coins, candy necklaces, and other hard candies. Add gummy ring half for clasp. Store loosely covered.