I've also had good luck with black shrink wrap tubing for bits that are a little too small for the rubber soft bits. The tubing comes in different sizes so you have to figure out which size you need. I wash the tubing in warm, soapy water, rinse and dry before using. You may have to use something to stretch the opening of the tubing to get it over the bit but after that you can slide it on as far as you want and then trim with an Exacto knife.
One other benefit is that the tubing is a lot thinner than the rubber softy bits.