I would be tempted to simply drill out a new hole in the spot that will fit your horn.. . . . The problem is that the "u" shaped clamp that supports the bow (so not the one that holds the end of the bell) only has a choice of fixed positions. Most of these positions are too far down to be of any use. And the one that's on the right position (the highest one) is positioned where the bell keys are.
If that bottom U clamp where to slide up and down, it would be fine.