Go to one of your local auto parts stores or Home Depot or other places like that and look for one of the el cheepo deep pattern 1/4" square drive socket sets, preferably 6-Point, that are often found on their $5 or $10 bargain table. From the set, choose the socket whose OD matches the OD of the special nut. Face the hex end flat. Mill the end back to leave two almost square or rectangular pins that fit the depth and width of the slots in the nut. You now have the special socket for that particular size nut. Keep the set together as sooner or later you will need one that is a different diameter. Eventually you will have a special socket set that cost $9.99 instead of $99.99.