I use that 12Volt blanket (more like an oversize heat pad reallly) but it
will keep you warm since it was not designed to be used with blankets but
rather as a seat warmer while driving.
A "couple of gel-cells" may not be enough. The blanket is without a
thermostat and runs continuously making the heat level hard to manage and
the power consumption excessive.

I am considering a cycling timer or external thermostat.

Help me out project people...

Rachel http://www.gnarlodious.com

Entity Marshall Ruskin spoke thus:

Why not consider using those 12V electric blankets under sleeping bags, and
have a couple of gel-cell batteries to power them