I have found that my favorite curing sphere is a sewn cricket ball. They are heavy and the perfect size for cupping the sling's pocket. After moistening the back of the pocket just a tad, I realized that cricket balls are expensive but three dimensional so why not capitalize on that?! Here was my solution.
Remember to store your sling wrapped around something round even if it's a ball of paper. It's good to encourage that leather-memory to maintain the subtle shallowness of your sling that allows it to be the most powerful throwing tool known to man, under two ounces... not a man under two ounces but the tool.