Add grated cucumber and salt to a sieve over a cup or bowl. Let sit for 5 minutes. Once the the cucumbers appear wet, stir or muddle to extract juice and set aside. Using a funnel, add gin, dry vermouth, filtered water, cucumber juice and Castelvetrano olive juice to bottle. Seal the bottle and give it a swirl. Store in the freezer until ready to use. When ready to serve, give the bottle a shake and pour into a chilled martini glass. Finally, garnish to your liking.