It may take a little longer but...
Make a base for the horn out of leather (moulded to the shape of the base of the horn along with a rim that sticks out perpendicular to it all the way around). Drill holes into the base of the horn and stitch it to the leather base. When done stitch the rim to the helm. That's pretty basic and doesn't need to use all sorts of chemicals. And you don't have to buy other stuff, cause I'm guessing you already have everything you need. And that will be super stable as well.
And with the stiching done on it, it will look really cool. :D imo of course!
B.