Get a hardened nut that will slip over the shaft. Grind away most of one end to leave two little ears to fit the slots. Use a socket to turn the nut. Fill the socket with paper towel so you are pushing the modified nut firmly onto the slotted fastener.
I have a lathe so I would get a solid round of iron or a thick washer and drill a clearance hole lengthwise. Then I would drill and tap for two screws to engage the slots. A washer I would weld on some pipe for a top hat look.
BilL D