Creating a plastic cap for a inflation valve on a whitewater raft

I am looking to create a plastic cap for a small inflation valve on a whitewater raft. This cap needs to be durable and capable of withstanding harsh conditions while providing an easy grip. The inflation valve needs to be small. Plastic material is ideal, though we are open to rubber or metal alternatives if the need arises. Ultimately, the goal is for the cap to be resistant to wear and tear while providing an easy-to-use grip. We look forward to your proposals!