McMaster Carr sells knobs in many sizes. (
www.mcmaster.com). Just go to their site and take a look.
I would suggest buying a knob from Mcmaster that fits the end of the threaded rod. Remove the cross handle, and slide the knob over the end. Then either use the set screw to tighten the knob, or cross drill and pin it in place with a spring pin
Sample Knob. This one has a #10-32 screw to lock it in place.
Spring pin sample.
And you get this: