the board Im using is 1/2'' thick, so it doesn't upset the car much more than any other bump you would have on a race track. I may bevel the edges of the board, which would lessen how much it upsets the car.
Im going to make a box on the part of the board that extends beyond the track, which will protect the phone in case someone goes outside the boarder.

The video showing how the app works shows a simple cardboard box turned into a ramp for a losi micro truck, and the phone is on the ramp and reads the vibration. So, it may be possible to use an even thinner board than Im using. I would try, but the 1/2 thick board was the only one I had on hand.
Any 1/10 RC is going to jostle any board pretty good, enough for most phones to pick up the vibration.