Mix equal parts salt and sugar on a small plate for the rim garnish.
Wet the rims of the margarita glasses with lime juice and dip in the salt-sugar mixture.
Place cantaloupe cubes in a blender and blend until smooth.
Strain the puree through a fine-mesh strainer to remove pulp.
In a cocktail shaker, muddle mint leaves gently to release oils.
Add cantaloupe puree, tequila, triple sec, lime juice, and agave nectar.
Fill the shaker with ice and shake vigorously for 15-20 seconds.
Strain the mixture into prepared glasses filled with ice.
Garnish with cantaloupe balls and mint sprigs.