Windsurfing Mast Mount for GoPro Hero.
More info here
You will need a screw and a wingnut, stainless steel, size 2-inch 10-32, sold in hardware stores. 10-24 is OK too, 10-32 is just a finer thread, a bit easier to tighten. Metric size is 5-cm long M5. This product and/or service is not affiliated with, endorsed by or in any way associated with GoPro Inc. or its products and services. GoPro, HERO and their respective logos are trademarks or registered trademarks of GoPro, Inc.