The "Nut" you need is called appropriately enough, a "J-Nut".....They are almost exactly as newt described. They come in different forms, for different sized hardware, different thickness materials and different lengths. I have the common sizes just for that purpose on those machines. Is it a 5/16 x 188 threaded bolt?? 1/2" head ?? Where are you located?
I'm in New Brunswick Canada.