I agree with making a wrench. Just cut on a bandsaw and file to fit. Alternatively, I have sharpened a prick punch and place it just inside one of the bumped-out "ears" that make up the hex. With a hammer make a light hit enough to make a seat area. Place the prick punch in the seat and turn it so that is now facing the direction you need to loosen the nut. Do not go completely on the tangent or else you will breakout the crater wall of your seat. You want the angle to be somewhat biased toward center and somewhat on the tangent. Finally, once you have that compromise angle, give the punch a good sharp hit with the hammer. Several hits might be necessary adjusting the angle between hits to reestablish the crater seat and to provide rotation. The shock and rotation often break things free.
Dan