I was looking, just yesterday, at something the Ryerson UAV team did, posted by Madhuran Wilson, that might interest you.
I believe he said that the foam caused the wind to impact on the flight, but there is no reason you could not use something thiner, like your drawing above, or maybe a thin CF cutout bumper or partial circle.
But he made this by printing the motor mounts custom. Still, I thought you'd like the photo:
ive been using just a C shaped plastic stick wich pointed out from the arm and was about 5 cm longer than the prop outside radius. worked and saved some props. now i dont need them anymore :) btw, by removing them i got slightly longer flight times.
There is a similar solution for the Gaui 330x: