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.
Screw drivers that large aren't too common. And the slot would be deep enough that unless you used a good quality screwdriver, the tines would bend rather easily. The advantage of starting with a hollow cylinder is that it doesn't matter how far out the shaft that the nut is threaded onto sticks.
I've used modified screwdrivers for years.have a large selection from small to large.never(so far) bent one.i use snap on or good ones..piece of tool steel with slot works too..just how I do it..works for me..
Lock the saddle to the bed and then cut a block of hard wood or steel and put it under the broken side for support and use a Pin Punch and one good wack with a 3 pound hammer should loosen it and then hand screw it off. If the saddle crank is in the way build around it or take it off first. If you have time and money mill up a spanner wrench. The key is to use a big hammer to hit it. Many use a lite hammer and all it does is bonce. If it dents the nut, thats what they make files for. Looks like someone dented it before. OH that is a right hand thread. So hit down on the left hand notch.