I’m doing an HPDE next week and want to mount my GoPro inside the cabin between the seats, just behind the little storage hole. Has anyone simply put one of the sticky mounts on that area to hold the GoPro there? I’m not sure if it will hold. Alternatively, I have already experimented with hanging it upside down from an extended suction cup mount at the back of the sunroof. But idk if the instructor will make an issue out of it.

 

Don't know what the instructor will say, but I have mounted it with a suction cup to the sunroof. Works very well. Just make sure you have a "bracket arm" long enough to get it low enough so that the rear view mirror don't cover too much of the camera view. If you set it right you will be able to read out both speed and rpm.
I had no major problems with vibrations at any speed. Be sure you mount it without the waterproof casing (get the right bracket if you don't have it). It makes all the difference regarding sound 😉
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Here is one of the videos I made. The camera should be placed a little lower and a little more to the left (more to the right on a right hand car):

 

Yeah I’ve tested mine out hanging from the sunroof and I get my hands, head, Speedo, gear, etc. so I may just do that.