To mount mine last year for our Hells Canyon trip, I cut a 12" section of Sch 80 2-1/2" PVC pipe (gray) which I clamped to the outside back corner of our cat frame for the base. For the pole, I used a 4' piece of Sch 40 2-1/2" PVC pipe (white) with a coupler and a 12" section on the end of the coupler. That slid very nicely and into the Sch 80 pipe base. Attached the gopro to the pipe near the top with one of the clamp mounts. Used the gopro remote to turn the camera on and off and to start and stop recordings.
Also tried to rig up a friction cable system so that I could rotate the camera from the front of the boat using parachute cord attached to a rubber tie down around the pole in back and tied to a swivel setup in the front, but it didn't work very well once the pipe got wet. So, we abandoned the rotation early on on the trip. I was still very happy with the video result, though. Here's link to my edited video that I posted on youtube after the trip.