Well,that would be one way to try ,but making a proper sand mold is a bit more complicated than that,and properly done using some balls you already have would result in very nearly perfect balls,depending on the coarseness of local sand.You could also make many balls with one pour,downside is-you only get one pour per mold build and the mold is destroyed when you remove the molded item and it's big and heavy and fragile so not portable,I know how it's done[in THEORY] and the safty concerns[must be totally dry before the pour]...However I"VE never done it and there are ways to screw it up ,but a few trys and...Small portable molds can be fashioned from wood,soapstone,cuttlebone maybe plaster of paris.I'm going to start a wood one today.