Those are called "coupling nuts". They're used to join pieces of all thread rod together. There are dozens of them available on Ebay in all sorts of lengths, threads, and materials, for a lot less cost than the ones in your link. It would be a very simple machining process to press one of those nuts into a knurled piece of brass round stock to make thumbscrews and eliminate the need for a wrench at all. Using a plastic thumbscrew with the same thread stud as the nut, and some red LocTite would be even simpler.
Jerry
