After grating the cucumber, press out the excess moisture by lining a colander with a paper towel, putting in the grated cucumber, and then topping with another paper towel. Squeeze the cucumber into the colander to push out the excess water. Do this several times (but not so hard that the cucumber turns into pulp). This will prevent your final sauce from being diluted and runny.
Mix everything together in a medium bowl.
Cover and refrigerate for an hour or so before serving.