I have used the backside of a spoon directly on the offending spot before with success. Just rub the backside of the spoon directly on the "styrofoam" with some pressure and you can deform the "styrofoam" to some degree. Remove the liner before you start so you can see what you are doing. As long as you are not trying to displace a large amount of material, this should work fine. It has worked OK for me in the past.
Disclaimer - I'm sure this voids the warranty and puts you in grave danger...