Welp, decided to go a bit of a different direction with the mounting of the batteries. I decided to use 1/2 polypropylene plastic instead of wood, and to use a much smaller piece to clamp it. Still same basic idea with the clamping though. Currently, my largest victory is getting a heating plate made and working properly! I used a piece of polished stainless steel:
I stuck qty 4 heating pads on the metal. Each are 7 watts, so over a 24 hour period is only 55 watt hours. Ideally, this should never turn on. We'll be full-time living in our van and have a small diesel heater keeping it warm 24/7. So really, this heating system is only active in case of an unforeseen emergency. I wanted something with as low of a power draw as possible, but still able to add more if I needed. As you can see, there is plenty of space to add additional heating pads if these 4 aren't enough. My main concern is whether or not the heat will conduct up to the top of the battery. They are pretty dense, so I can image they would.
I'm using a W1209 thermometer to trigger them on and off. Then, I'm using a repeat cycle timer to modulate the temp. I can set it to be on for 10 sec, of for 5, on for 10, off for 5, etc, and change whatever increments I want or need. My goal is to test the system on a cold day to figure out what intervals I need to set the repeat cycle timer to, then never adjust it again.
The temp probe is in a cup of ice. It is set to trigger on at 40ºF, ±2º. And actually, that ± is also adjustable with the W1209 thermometer under the hysteresis settings.
As is, the plate is about 80ºF-90ºF in open air on a 50% duty cycle (10 sec on, 10 sec off), roughly 60ºF ambient. I'm sure when it is 20ºF ambient and tucked under the (cold) batteries, I will need to adjust the duty cycle accordingly.