We use the size on the left of Bruce’s picture but our uses a 3/16″ allen wrench to install/remove. We use the 1/2″ long zus. We also use there thin plastic washer that goes under the head which protects the head from digging into the fiberglas. We get ours from JEGS.
You can get on Jegs (www.jegs.com) to see these. Put in the part numbers below in the search bar and it will pull them up.