Using pre-existing centre-fin holes, I drilled a hole for the bolt through the mount, bored around the top to allow the screw head to sit flush, then tightened the nut over a washer on the other side, hacksawed the remainder of the bolt off and capped it with a plastic nut cap underneath.
I have one fitted at each end of the board and have added a floaty back door and leash.
First filming today!