I've seen one Chinese manufacturer that makes a special very thin wrench that locks into the prop hub's outer edges of a particular brand of motors.

It is thin enough to fit between the space between a spinner and the cowl.

Very neat.

That gave me the idea to try a bike wrench I have that is similiar. This worked perfectly for a fairly large electric motor I own.

It would be somewhat easy to manufacture a flat adjustable wrench, with "C" arms thin enough to do the same for engines of different sizes... and grab each side of the prop hub to immobilize it.

Sell it for $20.00 and all of the electric flyers I know would buy one.

"why hasn't anyone done this?"