On the outer nut you should be able to get away with a 17mm open ended spanner, or even, whisper...an adjustable spanner![]()
I have an adjustable spanner. It was an apprentice made piece from the 1950's (not by me!). There is no play in the jaws and the adjusting screw thread is fairly fine allowing very precise adjustment. It is a lovely tactile bit of work and feels good when using it. I do use it on cone locknuts.