is that steel or aluminum ?
if it were aluminum, you would have more latitude of adapting
a pivot pin. steel is very unforgiving when it comes to hand tools only.
if it is steel, I would put on the wall and make a copy of it out of MDF,
plywood, plexiglass or other soft material that will work properly when you want it to.
if you REALLY want to keep it, if it were me, I would grind a tapered
recess in the slot and grind the appropriate size nut to fit the recess, then the pivot
point on the end of the threaded rod, and use some Thread Locker to hold it in place.
the tightening knob will have to have the hole all the way through for it to work.
and no, it will not always work properly every time. (again, hang it on the wall).