Hmm. perhaps you are right. Kinda ironic its too cold to cool your card.
I guess the obvious solution is move them inside and actually use the heat produced by it or at least inside a shelter of some kind that can warm up a bit.
Alternatively, Id consider making something even more ghetto based on cooler that has no heatpipes. Something like an old Zalman copper flower or VF700. If you have the room for it, even an old massive copper CPU heatsink. It might be inadequate at room temperature but I suppose most things would work at -15C
Only thing is it needs a small enough footprint to not be obstructed by any nearby components. Not even sure that I have any stock heatsinks floating around. 6:30am and way too tired to tear apart my room looking for heatsinks. I know I have a thermaltake big typhoon, but that uses heatpipes solely to transfer heat energy to the metallic fins to dissipate the heat too. I also believe I have a waterblock and an extra radiator, but no extra pump. z_z
I'll mess around with stuff this weekend.