D.P.
I found a small stainless steel O. (my son hates the super heroes too) and it keeps his food warm. I got it at Target last year. This is it:
http://shopthermos.com/detail/TMS+2330P
Do you pre-heat it with hot water for about 5 mins before you put the food in? That's what I've been doing and it works.
I can get it in his soft side lunch box along with a drink and ice pack.