EDIT - DO NOT ATTEMPT THIS TECHNIQUE - IT DOES NOT WORK. See this post for why.
My assistant Naiche came by yesterday and helped me install the warmers. First step is taking most of the batteries out - the warmers fit pretty tightly and it is very difficult to shove them between the batteries. So, out they come, and in goes a warmer:
Here is the rear box, all complete (you can see the pink foam peeking out between the batteries):
I had to drill a new hole and install a grommet in order to get the cord for the middle box to come out in a useful location:
All three warmers are hooked up to this terminal strip, with the input 120V AC line hooking up as well:
Finally, installed an inlet just like the 240V inlet (except with a 120V 15A plug) and wired it up. Here it is, plugged in and warming:
Using a clamp-on AC ammeter, I determined that it is pulling about 5A, which is about 600W, which is about what I expect from the warmer cable I'm using. Not a lot of heat - but hopefully enough to maintain the batteries at a warm temperature. It's very cold out right now, so I'm warming the whole garage with a 4000W electric heater right now. I'll let the batteries stay at this temperature for about 48H and then see if the warmer can maintain them in the cold.