I like mine in his feed tray/manger. It also helps him slow down when eating.
In the manger is the easiest to me. Also my horse loves to play with the rock, he makes it roll!
You can put it is the manger, it is true that some horse make it drop. You can put it at the height of the wither or mouth!
Most of the time I put it in the manger as my horse always makes the rock fall!
I put it at the height of the horse's wither.