Wooohooo!! Would it be possible to put one of your temperature probs in the peanut oil, to see what temp its buffering at?
I've been in contact with PCM Products, who makes a variety of Phase Change Materials for buffering temperatures in homes, fridges or computers. The A4 material which buffers at 4 C and is made from "organic materials" costs about £4~6/kg when purchased in 1 ton quantities. I believe the price variation depends on how it's encapsulated, ie what type of ice packs you have them put the fluid in.
Of course peanut oil is about $37 for a 5 gal/35 lb container...so this could be a much better DIY solution. And I'd much rather eat peanut oil on hand to eat then some random unknown fluid.