I take a ELK Power Howler, remove the mouth piece and modify. Then I carefully attach them to a modified horn. Not an easy task. I learned quite a bit working with these. It's a real balancing act. A step in the wrong direction, and the horn sounds awful! The mouth pieces alone cannot be bought unfortunately. -But then everyone would be doing them.
I've seen some good ones. But I've also seen some poorly made one. The concept is akin to a turkey tube call using a latex band.
Dave