I had the same problem and it cost me new gears in the end but I found a solution.
Put the nut in a lathe and drill out the thread with a 14mm drill, then machine a recess of 0.5 x 52mm to get some extra clearance and machine a 2 x 42mm pocket for the SDS-click nut to fit in.
I did this about 6 months ago and use it professionally and haven't needed the spanner yet, hardly ever needed the locking pin (mine has a brake) and it never came loose.
Note: chisel marks were result of frustration to need to remove the nut.