Sounds like aerogel and a frozen slush of salt water would work, but you're correct about expense! At this point the problem has left the theoretical and has entered the nuts and bolts real-world engineering phase of "what exactly can we live with" vs "what can we afford". These will mandate the tradeoffs and compromises that will result in your final assembly. Not knowing anything about your business I can't help you there ;0), but I can wish you luck!