What about dropping into each cell a coffe heater? Of course, with a shape that enters the cell access hole, instead of the large loop shown in the following picture. I've seen a couple of these that seem to fit in the access hole. You need six of those (if your battery has six cells) and an extension cord with six outlets.
You might have to bend it, looking for a shape that enters the hole without interfering with the top of the plates. I would bend it at 90 degrees, so it enters the hole and is positioned more or less flat on top of the plates, touching them, but keeping the heater immersed in the acid fluid. I don't know if the liquid is deep enough, over the plates, to keep the thing immersed, but you could add a little extra liquid if that's not the case, retiring that extra fluid before closing the battery.
I think it would be fast, cheap, reliable. As you are using six heaters, it'll take 1/6 of the time of heating the liquid with pipes, tank and one heater.
You would be using convection instead of radiation, so you don't have to cope with insulating air or whatever.
You need an external source to heat the thing, but I guess you can use an extension cord from the pit to the grid and keep the heaters inside the batteries up to the moment in which the mechanics have to retire (they take the heaters out, remove the excess liquid, if necessary, put the caps and that's all).
I'd use six thermometers to check the temperature inside each cell and insert/retire the heaters accordingl. You have to create the immensely popular position of "battery heater chief mechanic",
for him to avoid boiling the liquid, of course.
PROJECT FOR NEXT YEAR: Build the six heaters integrated into the caps. Power them with the braking power (if enough) and include a thermostat for each one to keep the optimal temperature inside each cell. Who cares about the temperature of the case? What's important for the chemical reactions is the liquid and plate temperature, I guess.
Btw, thanks for calling me a "young man"...