Hmm... you're right. With the non-powered rotors it's impossible. But you could achieve it with some diagonal walls, but then the area would be a bit smaller.
Treazer, can this be done without changing the level? Because if you add wall at the border, it will not be very nice either and the wall will make it even harder... Otherwise I agree, the tiny white line around rotor is not nice, but I am afraid it cannot be changed here.