[Gnawrocki1]

When using a castellated nut tool, especially with a lot of torque on the wrench, you need to make sure that you are properly piloted on the nut or it can slip off and damage the castellations (nubs).  

I could not tell by looking at the tool picture if it had a piloting feature.

I have not tried it but I would think a hose type clamp would work if you can grab both the nut and tool OD.  If you have room, I would add that to your procedure.