Sounds like a variation on a leather stitcher's vise. You use a leather strap- as wide as you want to hold the horn, and attach one end to the pedal. The other end is put through a slot( visualize the slot in the stock of an M-1 carbine for the sling) in the wooden post, or " anvil", which has either a "V" notch in the top, or a shallow " U " notch to help hold the horn. Often you see the strap fixed at the top end with nails, or screws, on the Right side of the anvil, then a loop is formed and the other end goes from right to left through the neck of the anvil in that slot, coming out the left side, as you sit and face the anvil, then down to the foot pedal which is under the left foot. The wider the diameter the horn, the higher the pedal is in relation to your foot. The loop has to be fixed to the top of the anvil in some way, to maintain the loop, and keep the horn centered in the cup or V. I saw one once- only once- where a mortise was cut out at the center of the cup at the top fo the anvil, on a 45 degree angle to the left side of the wooden anvil, so that the strap made a loop at the top to hold the horn, and the strap easily extended down to the foot pedal. The owner had used beeswax to lube the shaft of the mortise to make the leather move more easily in the slot. The wooden anvil( Post) was a 4" x 4", and the mortise was only 1" by 1/4". He was using a " Retired" piece of belt.
If the belt or strap is too short, use rope or cordage to extend or adjust it for length. Or splice, sew and glue another piece of leather if you want to go to that much bother.
These Horn vises were affixed to a bench commonly seen with shaving horses, only the length of the bench was much shorter.