I had one of those 2" thick styrofoam boxes that are used to mail frozen foods that I used as a hay box. My stock pot fit in it perfectly. Alas, the box disappeared sometime in one of the last few years moves. But now I just wrap my stockpot in a sleeping bag.
I haven't tried wrapping my rice cooker insert in a quilt or bag, but seems like it would work just fine. Just use one of those stretchable silicone lids to cover the insert, wrap it in an insulated cozy, and maybe drop the whole thing in a suitable cardboard box.
I'm sure those commercial thermal cookers work just fine, but they seem too spendy and single-purpose to me.