I've tried a number of hold-down devices. For the last couple years I've been very happy with a chain that is connected to the stump on the far side of the anvil, comes over the face and and has an 8# sash weight on the near side. It doesn't give an utterly rigid grip, but it's enough to keep the work from flying, very fast to use, never lets go, and leaves my hands and feet free. I usually just leave it draped over the step or the horn, depending on where it's least in the way.
It made it possible for me to make this hatchet alone using a 12# sledge: Prospect Hill Forge - StillsonHatchet