Fossil Imprints: Failed Recipe

During a recent hike with the kids, I had them collect items from the trail so we could make fossil imprints when we got home.  The recipe I tried really wasn't good (2 cups flour, 1/2 cup salt and 3/4 water).  I'd love to try this again but with a better recipe.  The hunt is on!  If you have a recipe, please share :)