Hi,
As mentioned in this thread ([url]seat height]), I used a wrench coming from an old BMW motorbike tool set and which is used to adjust the spring preload of the rear shock absorbers.
It is similar to this one:
I suggest you to go to a motorbike/snowscooter dealer and ask him for such a tool. But prior to that, measure the diameter of the nut as well as the width of the notches.