Spray a panini grill or griddle with nonstick spray. Heat over medium heat. Arrange half of the cheese over the bottom piece of focaccia. Arrange the prosciutto, arugula, and then the cantaloupe atop the cheese. Cover with the remaining cheese, then the focaccia top.
Grill the panino until the cheese melts and the bread is crisp and golden, about 6 minutes. Cut the panino in half and eat.