I don't think it's a dumb question at all. I normally roll a full sheet of printer paper into a cone, like a dunce cap. Some scotch tape keeps it from unrolling. Use the sharpened tip of a pencil to open up the pointed end of the cone enough that powder flows through it easily, but leave the tip small enough to fit into the horn tip. Works for me.
It seems to me that the removable tip of a doctor's or audiologist's otoscope would make a perfect powder horn funnel, but I've been unable to get one and try it. I've asked a couple of times during office visits, but was told they sterilize and re-use the tips.
There was a photo on another forum recently of an antique powder horn the had a threaded knob in the base. The wooden knob was for attachment of the carry strap, but it had a threaded shank that screwed into a pretty good-sized threaded hole in the base. It would have been easy to fill the horn through that opening in the base.
Track of the Wolf sells a brass fitting that functions on the same principle. It has a brass knob with a threaded shank that screws into a threaded bushing you can install in the base of the horn. It has a rubber O-ring to seal it tightly. The really clever part, though, is that the thread pitch is the same as the threads on large powder flask spouts... and the powder flask funnels made bY Treso and Pedersoli. Probably a modern innovation and not authentic, but practical.
Notchy Bob