Go to a hardware store, ace carries it, and look for Envirotex Lite. It is food "safe". There is no epoxy "approved" for food contact. This is as close as it comes. It is what bar tops are made of. Mix up a bit and put it in the horn. Sit and roll the horn keeping the stuff rolling and coating evenly. After about an hour it will finally set up and you can set the horn down to finish drying. A couple to days later it is ready to wash out and drink from. This is what I put in my drinking horns.
We have a group horn that is used for drinking circles. It holds a little more than 2 bottles. It is natural with no coating. People say the it "mellows" the mead when drinking from it. It does affect the flavor some. Mellow is as good as any word.
I have 2 of my own horns. One is about 12-14 ounces. The other is 4 feet tall. It holds 5 bottles of mead! I only used it a couple of times. More for show than use.