Another thought for stabilizing: Add a small-link chain (either nearly weightless plastic, or metal if concerned about breaking in cold climate) to your kit. Place an S-hook in each end link. Wrap one end around the tripod's spider platform and hook to a link (or use the column's hook if there is one). Let the chain hang down so you can create a stirrup under your boot, using the same technique as at the upper attachment. The height of the stirrup above ground is then adjustable; it should only need a slight ground clearance. You can then press downward with your boot to stabilize the tripod and there's no need to go looking for rocks. Stretchy cords are not as effective as chain links. The length of chain needed to satisfy your highest tripod use can be determined at home prior to heading out. Hope this is all clear; it's easier done than said.