If a pool noodle is what I think it is (it isn't a common expression in the UK!), foam-based floating support, then I'd think it would make an excellent insulator with all the air trapped in it. That was a good experiment and I'm surprised to see that ambient warms to such a degree - 1.34W seems like a lot. I know of some transistors that wouldn't hold up as well as that ice.
It's easy to forget that the air pockets make for a large surface area. Your graph, for the 4 degree point looks fairly linear - was it like that all the way? If not your thermal resistance is just a snapshot in time isn't it? There is another option: most domestic fridges are set at around 4 degrees (perhaps 5) but it may not be clear or exact (mine has a circular knob that can move between "cool" and "eco" but without degree C markings.) However, it should be constant as long as you don't keep opening the door. If you can measure from within there - e.g. stuff your DMM, thermistor and flask in the fridge then ambient would be controlled - you could also use a second thermistor to measure how constant that fridge ambient was (how much hysteresis does it exhibit?): you could verify that thermal resistance!
If nothing else, these are 'outside of the box' experiments with thermistors and good fun.
