My first version is pretty rough but it works pretty well. The coil is a piece of 3/16″ plated steel rod, I’d guess that you’d need 3 feet. A good two inches of each end is inserted into drilled holes in the handle while the middle loop is secured with a screw eye. I first opened up the screw eye and screwed it into the handle, and then inserted the loop into the handle. Then the middle loop was moved into the screw eye which was bent closed.
In retrospect it looks like the commercial version use a smaller diameter rod which would make it easier to mix the dough, this thing pulls pretty hard. But in the end this coiled shape mixes the dough very quickly and gets all the flour off the sides of the bowl.