Product review (4 stars): Easy fix for loose knobs
I have a Jen air stove and the center-hole in the knob is too big. But if you take a large nylon wire tie and cut it to 2 1/4" them folded it in half (roughside in) then push it into the center of the knob (see picture) then spread nylon tie and push knob onto shaft on stove for a tight fit.
 I gave it four stars because I had to modifier it to fit.