For a while I used a handlebar extender mounted on my offside fork near the hub to give a better perspective by including my right leg and pannier in the shot. ( I'd been told by police that a recent close pass was impossible to judge because the footage showed no part of me. Bollox of course, but I had a spare Fly6 so tried this as an experiment.). It used two clamps as a single clamp allowed a bit of vibration, spoiling the video.
You can get them from places like Banggood or
eBay or Amazon, e.g.
https://www.amazon.co.uk/Handlebar-Speedometer-handlebar-extension-Motorcycle/dp/B06XCN5W8G
Is there any way to fasten some sort of short post to the child seat?