"I just had the vision of tightening everything up with epoxy on it all and the possibility of twisting the threads. And because it's been ground flush, the sides without threads would twist into the slot then make the rod loose and disaster"...
Great idea and no weld tutorial, thanks OP!2thumbs
I wasn't asked but here's my thoughts addressing your concerns FWIW. :bud:
You could Just make the slot a bit more narrow eye ball or mark the area out on the threaded rod then file that section down to fit the slot nice & flat on both sides of the rod so that the all thread will still friction fit in the slot then if you want to you can file or grind off the threads that will stand proud of the tang flat, which were ground off in the tutorial.
now you have a nice tight friction fit Rectangle locked in the slot by the groove in the tang, and if you do it right it will likely break the thread rod before it could twist out of the slot.
if you want to you can use the original size slot and use a larger diameter threaded rod and then modify as explained above for the rectangular slot, if you combine that and strong mechanical fasteners, along W a strong handle material it should hold up to a whole lot of punishment W/O coming loose