Wooden Dowel
Go to Home Depot and get a wooden dowel to drop into the slot of your mast. Cut it to the length you want so that the goose neck never goes lower than you want it to. Remove the goose neck, drop in the dowel, replace the goose neck. To keep the dowel from coming out when you trailer, use a small stainless screw and washer to clamp the dowel to the mast. This costs about $5.00