These may look like a great idea but they're too soft. You will still be able to break a split shaft pot very easily if you tighten a set screw too much. Using a piece of plastic inside the shaft is a much safer way to "convert" a split shaft pot to solid shaft. Also, with many knobs the pot's diameter will become too large to fit inside the knob with these sleeves attached.
Not recommended, a waste of money. Buy the right type of pots instead, or stick some thin plastic inside the shaft if you HAVE to use a set screw knob with a split shaft pot.