Line the rim of small glass with lime juice and dip in coarse salt to coat. Then add the mezcal, tequila, and spicy margarita mix. Give that a stir and then fill the glass so it's 2/3 full with ice. Pour the watermelon juice over the top, and if needed you splash on a little more than 1 ounce so the glass is full (this is why I like to add this last)
For the spicy margarita mix, simply combine 1 cup lime juice, 1/2 cup granulated sugar, 1 cup of water, and 1/2 teaspoon cayenne in a small pot. Heat over medium heat and whisk constantly until the sugar had fully dissolved. Store in a jar or container that can be sealed and refrigerated.
For the fresh watermelon juice, simply blend watermelon pieces and then strain through a fine mesh strainer.
For a frozen version, just add the above recipe plus another handful or two of ice to a blender and blend until smooth like a slushy.
You can use all mezcal as opposed to 1:1 mezcal and tequila. It's up to you. I just like to mix them as it usually tastes a bit more balanced to me but it also depends on the type of mezcal and tequila.