First, you can look at the first picture I included of the bottom of the battery pack and see how long are each of the thermal pads I used. You can find a variety of forms and lengths for thermal pads.
The ones I have used had a sticky bottom so I could stick them directly on the metal following even the shapes of some metal channels in the bottom plate of the battery pack (look at the first picture mentioned, you see the orange thermal pad following the shape of the channels). I have added red RTV high temperature gasket maker on the side of the thermal pads to make sure they would stick in place.
Just make sure you can locate them between the screws and the plastic clips (both types used) that retain the plastic protector sheets under the battery pack. With 2 battery warmers (12V lead acid battery warmer, 80W each, working on 120V), 4 thermal pads (90W each, working on 12V) and 5 thermistors to locate under the battery pack, I had to make sure I was not putting them under a fixing hole for the plastic protector sheets.
Second, pay attention to your series thermal pads arrangement.
As it has already been mentioned at the beginning of this post, if you power two 120V thermal pads in series with a single 120V supply, you are doubling the total resistance and dividing by 2 the total power of the thermal pads.
Each one is getting the quarter of the power it would had had if not put in series (P = V**2/R, if you divide by 2 the voltage for the same pad resistance then you get the quarter of the power).
If you plan to buy 240V thermal pads and supply them with 120V, don't put them in series because you already have half of their nominal voltage. With 120V on a 240V thermal pad it will give you a quarter of the heating power for each thermal pad. I doubt that it will become very hot with only a quarter of its power.
You will see however that it become costly to "lose" this available power as you must install more thermal pads (in parallel) to achieve a sufficient heating power (or the one you are choosing).
You are right about the hot spots. Temperature can go very high on thermal pads (even above boiling temperature, depending on your thermal pad). The good thermal pads manufacturers recommend a thermal control. The only thing keeping the temperature at an acceptable level is the huge thermal mass of the battery pack itself.
The thermal pads stuck to this huge mass dissipate the heat in the metal bottom plate of the battery pack and probably not on the battery cells directly (each battery cell is itself in a metal box).
To avoid hot spots, I have built an electronic thermostat to regulate the temperature at very low levels. The battery warmers start at around 28 degrees F and stop at about 32 degrees F. For now the 4 thermal pads I installed are still unused. They were planned to try to maintain the battery temperature while the car is unplugged for a long time at work. Still have to work on this with a big 12V battery and thinking of its recharge. My intention is just to avoid the battery to freeze.
Two days ago I had a case of my 2 battery warmers being started (160W of heating power) continuously. I make a led blink to know for how much time the battery warmers are/were started. After returning from work with battery temperature with no bar, I plugged my home made battery heater. I saw later in the evening that it was started for 640 "approximative" minutes (more than 10 hours) and later saw a strange blink (I have limited the display time to 999 approximative minutes). Outside temperature was between -2.2 degrees F to -9.4 degrees F (-19 degrees C to -23 degrees C). Had only 1 bar of temperature while heating the battery at this time. The lowest temperature sensors in the battery were about 15.8 degrees F (-9 degrees C). The led blinking went back to normal later after the battery warmers have been stopped by the microcontroller.
You certainly don't want to have the foil-faced bubble wrap to isolate your battery during the summer. It is not a problem for me in Canada.
About the very approximative temperature inside the battery, you can check :http://www.electricvehiclewiki.com/wiki/battery-d1/
Hope this help.