After burning up a couple drill bits trying to drill through all the stainless all-thread shafts I decided on a different tack. I simply jammed lock nuts up against the nuts in the knobs. So far it is working. I probably didn’t need to use lock nuts, but felt it would help keep the knobs from coming loose, at least in one direction. If this ever fails I will try gluing the knobs to the lock nuts. That should be at least as secure as the connection to the cover.