The Insta360 X series cameras (One X, One X2, and the new X3) are fantastic action 360 cameras, but the form factor makes chest-mounting awkward (see below).
A simple mount rotates the camera and places it much closer to your body for more stable footage. It's held far enough from the body to prevent scratching of the inner lens, but close enough to feel a real difference compared to the standard vertical mount.
The mount is available here, and is a 3D printed ABS part which is plenty strong and field-tested. It's a simple, yet effective solution!