3/8” square rail risers are sold individually or in a 10 pack and only available in gray. It is designed to create better airflow and help the water to drain off the deck which helps prevent mildew. They are designed with lips on the edges to help center the spacer on a 1-1/4” wide railing. Rail risers are also known as fence risers.
We use this product in house on our re-decks and highly recommend it.