I know this might sound daft but why not get one of those insta-heat bags and wrap it round the carbs / wedge it between them if possible. We use them all the time at rugby but i can't remember the name of em.
EDIT:
Found em, Boil the bag for 5 mins or so, last upto 4 hours methinks. come with a velcro starp.
http://www.firstaidwarehouse.co.uk/p...=hot_cold_ther