You can glue more foam on the inside of the case, maybe weave the heater wire through a layer of the foam. Even some on the outside. Bubblewrap too is a good insulator. Where i used to live the ummm "Europeans" used to wrap their outside hot water systems in bubblewrap to save electricity.

The best option would be to use the hard case with better insulation, and you can put the battery in the hard case. Then make a good shoulder strap etc for it to fix the ease of carrying issue.

 

Hi,

a couple of years ago we tried to enter the underfloor heating market. We produced a product which is stranded resistance wire covered in ETFE which is a tough polymer rated at about 155°C if I remember correctly. The resistance is 11.93 Ohms per m. I happen to have some left over.

You should be able to get a workable solution with this. 3 pieces of 3m each wired in parallel will give you 12 Ohms drawing 1 Amp and producing 12W.

Play around with some configurations, when you know how much you need PM me with a postal address and I will post the wire to you.

Cheers
Andrew