From the limited experience I have with Insolia (I have a pair in a pair of heels that I don't wear very frequently), they're pretty good, but if you're looking to push your foot back in the shoe (which is definitely what you need to reduce heel slippage), you're probably best off going with something like
Foot Petals' TipToes.
They'll give you some cushion under the ball of your foot, and push your whole foot back in the shoe so the heel should slip less.
Plenty of companies make these, but the Foot Petals ones do happen to come in a variety of fun colors and patterns.