I have some 4mm ply. If I cut the picavet out of a 10" square piece double thickness and then drill holes along the arms to reduce weight, I could then use small galvanised eye hooks instead of pulleys.
The GoPro could lie face down on it with the lens poking through a pre cut hole. Some small half inch side strips forming a square into which the GoPro sits could be lined with some thin foam pieces to act as a cushions. A small backing plate strip with foam backing could be secured across the back of the GoPro with lightweight butterfly screws and nuts. An alternative could be some Velcro straps that have been screwed to the board. The whole lot could be attached to the kite string about 30' down so that the kite already has some lift in it when picavet is attached. I could use simple lightweight carabiners and twist the line around them several turns.
Alternatively, I could just drill through the centre where all arms meet and push through a 5mm bolt and screw and glue a GoPro mount to that. This option would allow me to alter camera angle better. Sounds simpler. I could also leave the backdoor floaty on as well. Wouldn't be much protection for the camera other than the waterproof housing though.
More musing needed......cuppa, toast, marmite........chair in front window overlooking the valley, village and moors.........