from what I saw, many ppl, not just form horseworld, have a harness to mount a camera on their chest

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For example:

https://www.amazon.com/AmazonBasics-Chest-Mount-Harness-cameras/dp/B01D3I8A7A

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I would also advise against mounting the camera on a helmet, you want your helmet to be as smooth as possible, so any blows have higher chance to deflect and not get caught, it may be dangerous for your neck spine. even vikings didn't had any horns on their helmets :)

and if you are wandering, by a blow that can glance, I'm talking about hitting a railing, log or a tree stump when falling, I don't expect anyone to wear a equestrian helmet into a medieval style combat (or, Sun Tzu forbids, modern one! ) :D

Off Amazon - the exact one I bought isn't available anymore but they're all about the same.
Here's a cheap one:

https://www.amazon.com/AmazonBasics-Chest-Mount-Harness-cameras/dp/B01D3I8A7A/

The official GP version is only a couple bucks more:

https://www.amazon.com/GoPro-Junior-Chest-Mount-Official/dp/B00F19Q2Y8/