As already said above, you can find captive ‘track’ nuts in the standard coaster and IGH 3/8” size. I wasn’t aware that you had a ss coaster hub, as I’m not familiar with this bike beyond the fact that it’s a folding bike. That’s my mistake for assuming it was a hub with a thread on ss freewheel. Nonetheless, all of my advice still stands. Most important of which, is that you should be using captive nuts. Cheers.