Put the raw horns in a carboard box, and cover the horns with Borax detergent. It will kill the odor by killing the bacteria, and will dry out the core of the horn in the process, so it can be more easily removed. Save the Borax. It can be reused. You may have to run it through a screen if you get debris in the Borax from the core, but that is a fairly simple thing to do.
You will probably have to boil the horn OUTSIDE, and DOWNWIND, but if you put baking soda in the water, even the smell will be vastly reduced if not totally eliminated. Boiling will loosen that core so you CAN remove it. You can even add some liquid detergent to the water, to help in freeing that core, and killing any remaining bacteria that is giving off that rotten smell.
When you remove the horns from the boiling water, hook and pull out the core as soon as possible, and get rid of it. Bury it, put it in a plastic garbage bag- whatever-- but get rid of it. They do make decent fertilizer, but then so do rotting fish! Then, give the horns a coat or two of protective oil, on both the inside and outside. Be liberal in how you swab oil on the inside. This will help keep the horn soft to work on, and keep it from checking or splitting. The oil can be removed when you finish scraping the insides to get it smooth, and round for your base plug.