My best advice is either try the solution just above with dry ice, or forget about ice cream. For ice cream to stay firm, it needs to be much colder than 32 deg F. I have filled my Engle with various ice cream sandwiches/bars for the trail, and had to turn it up to almost full to keep it firm. Keeping foods frozen is much easier than ice cream. I measured the temp in our deep freeze at -1 def F. Our kitchen freezer was about 5 deg F. Be careful with the dry ice, as I have heard that you can freeze ice cream solid with it if you don't have some insulation (newspaper?) around it.