I think that I would use just a glued in fitted Oak wood base plug and peg the horn to these, as a powder horn would be pegged. But before attaching this plug with glue, pre-drill the inside center of the Oak base and use 1/4" diameter carriage bolt (with head on inside) to anchor horn thru your rack. Countersink the carriage bolt nuts on the back of the rack and your ready to go! No need to fill them with anything, as their tougher than you can imagine.
Rick