If you mount the camera below the rim of your helmet then you’re going to have a really bad time.

Mount a wedge to the helmet and the camera base plate to the wedge. I’d use wood for simplicity of fabrication. Throw some paint on it.

If you don’t have access to a wood shop then literally take your helmet into a big box hardware store and look around for something that’ll work as is, or to a small hardware store paying a little more for assistance in figuring out a solution.