How to fix the issue (gap) between bolster and handle?

[Horsewright]

Having worked with a couple of mountains of horn, I'd fill it and sand. Ya could get fancy if you have extra horn about and mix some horn sawdust into the glue before filling. But if not just motor on. Horn curls sometimes. Sometimes cause it wants to go back to its original shape zand sometimese because it was overheated during sanding. Problem with the later is it will show up later to bite ya. Some horns have more tendancy than others to curl.