Back in WW2 they used to have heated flight suits for the crews
of military aircraft. I imagine that they were particularly
useful in the high flying bombers with huge open gun ports on
the sides. I had a pair of the slippers at one time, and found
that they really did work. I used a variable DC supply to power
them. Originally they ran on aircraft power (28V) through a
rheostat. I never tried them in an airplane, but something similar
may be what you need. I have seen some battery powered socks in
outdoor supply catalogs.

Another approach might be chemical heat pads. I happened to look
at some in the hardware store the other day. You activate them
by simply opening the package and exposing the pad to the air.
Finding out if they will do the job will be cheap - they only
cost a dollar or so.

David Johnson