Spread the salt on a small plate. Moisten the rim of a 10-ounce glass with the lime wedge and upend the glass onto the salt to crust the rim.
To a cocktail shaker, add the simple syrup, tequila or mezcal, grapefruit and lime juices, and 6 to 7 ice cubes. Shake for 10 to 15 seconds. Pour the drink into the salt-crusted glass, ice and all, and top with Squirt or one of its stand-ins.. (If you prefer, you can strain the mixture into the glass and add fresh ice cubes before topping with Squirt.)
To make simple syrup, in a small saucepan bring equal parts water and sugar to a boil, stirring or swirling the pan until all of the sugar is dissolved. Remove from the heat and cool. Store in the refrigerator. It will keep for a month or more.