heating knob switch keeps falling out?

hello, i have a efa600h cooker, recently i took one of the knob switches off from the front, just so i could put a bracket over it to stop my little girl from touching, when i went to put it back on it was very loose.when i took the other one off it had a little black square thing attached to the metal rod in which the knobs slide on and this prevents them from not coming off, my question is, how do i fix the loose switch? and can i by these very small brackets at a shop?