An alternative to the method recommended by Zd542 is to wrap the shafts with plumbers teflon tape. Keep adding wraps of the tape until the knobs fit tightly. The teflon tape has no adhesive so it's easy to adjust or remove if needed.
This trick also works well with IEC power plugs that don't fit snugly into the socket.
This trick also works well with IEC power plugs that don't fit snugly into the socket.