If you use an old belt, well softened with oils, to wrap around the body of the horn, you can clamp the ends in a vise and get a pretty good grip on the horn. There are also rubber belts available in home stores, and hardware stores that can be adapted this kind of purpose.
If you take a 2 x ? and cut an arc in it larger than the diameter of your horn, then line that arc with the rubber stripping. That will provide a soft, rubber base to rest the horn on. Now to lock it in place, you need to attach a long strap to one side of the wood form, so that it loops over the top of the horn. On the other side, you need to have some place to hold the strap with a clamp. Drilling a large hole through the 2 x with a key hole saw so that you can fit the nose of a C-clamp inside that hole, and the other end of the clam against the strap would be one way to make this, and be able to "Quick-release" the Horn to move it. I think some kind of ring, like a saddle cinch, would be handed, so that you could pull up on that strap to tighten it on the horn, while you close the clamp( I would use vise grips, not a C-clamp).
For long horns, you would be served by some kind of stand, or "Cradle", with the strap located away from the area being worked, and a support piece with an arc to support the horn under where the filing or carving is being done.
CAUTION: I hesitate to recommend something different than what is shown in the Sibleys' book, simply because its way too easy to put too much pressure on a file on soft horn, when you can use both hands on the file. That's a fast way to ruin your prior work, by filing too deep. Having to hold a horn against a vertical post, or on a form shoved into the horn, while you hold the file in the other hand, makes you take your time, and reduces the chances of "operator error" ruining a horn. You constantly have to check the thickness of the horn where you are working, to avoid cutting into the middle of the horn.
