Place 3 ice cubes into a pint glass. Place 4 thick cucumber slices into glass, top with lime juice and simple syrup, and muddle thoroughly until cucumber pieces are juicy and broken up, about 10 seconds.
Fill pint glass about 3/4 full of ice; pour gin and elderflower liqueur over ice. Shake gently and strain into a cocktail glass. Add 1 or 2 small pieces of muddled cucumber. Garnish with 1 slice of cucumber and add splash of club soda. Serve with straw.