A lot of people use a leather bag with the rough side out, to give more "grip" to the surface of the horn. The bag is filled with lead shot. The weight of the shot holds it in place. But, unless you find a way to strap it down, work is still one-handed, with your other hand holding the horn.
Using tapered wooden or cast plugs shoved up inside the horn base, and then clamped to a vise, or workbench, allow you to use both your hands on your tools. :surrender: :hmm: :thumbsup: