Have you ever used one? I carry a foil blanket or bivvy bag in the bottom of my rucksack when I'm hiking or climbing. It was put in to action overnight during a mountain marathon last year, and I have to say I was stunned by how poorly it performed - managing to trap perspiration while allowing heat to escape

For the OP - would a blanket not suffice? I think any electrical contraption is likely to cause problems somewhere along the journey. Also, make sure you sit away from the emergency exits.