Frozen Watermelon Margaritas

Place watermelon on a large baking sheet and freeze until hardened, at least 2 hours.  Combine salt, sugar, and lime zest on a small plate and stir to combine. Use lime wedge to wet rim of margarita glasses then dip rims in salt mixture. In a blender, combine frozen watermelon, lime juice, tequila, and triple sec and blend until smooth. Pour into rimmed glasses and garnish with a watermelon and lime round.