My sheep book had handy advice for making a salt holder, this is a modification of the book’s design, because I had an extra piece of 1×8 laying around. You just cut off a square from the end of the board, split the rest of the board in half, and cut those two pieces into sides and angled edges to hold the box up. Nifty!
The sheep immediately went for their new salt, they seem to like to snack on it midway through their grain meals.