There are plenty of awesome GoPro accessories available on the market. If you have access to a 3D printer, you could always print your own accessories. Take the Deck Cam: a GoPro accessory that lets you mount your GoPro to the bottom of skateboards. It was printed using Tough resin, so it is durable. The mount accepts GoPro Hero 3 and Hero 4 models.
The above video shows how the Deck Cam works. More tips and instructions are available on formlabs.com.