It's too bad he couldn't have an simply automatic grid at low speeds and once going the grid pulls or rotates out of the way for better efficiency.
Actually a rotating grid may work using the air as its triggering force, once the air speeds up some sort of venturi force enables/disables a safety mesh across the opening.
"The propellers, which are currently largely exposed, will also be fully covered in a mesh to ensure limbs don't get too near the blades."