Nothing wrong with modifying tools to get the job done, or having a custom tool made. Could you cut/file/grind off the parts that are interfering on the outside of an 18mm or 11/16 wrench and enlarge the jaws slightly so they fit the nuts better? Removing metal will make the wrench weaker but if you start with a high quality tool it may still be strong enough to break the nuts loose without damaging anything. Having a custom nut splitter made would be another option.
