There was just enough space, with a bit of Dremel work to fit the buck converter I had (from Radio Shack). It was tight but it just fit with the wiring. I've honestly never pushed my packs to the limits because I haven't really needed them weather-wise (hence my reluctance to pay full pop for Thermics or Hotronics).
1 vs 2 vs 3 batteries with or without a converter all comes down to the power, current, voltage, resistance relationship that follows Ohm's law. You need to start with one or two variables and work the rest out from there. It's hard to say what is suitable on one piece of the puzzle only.
i.e in the Thermics, the resistive elements are about 10 ohms, and the middle setting of the battery pack is about 1.8V (IIRC). That gives you a a current draw of 1.8/10 = 180mA for a power output of (1.8)^2/10 = 1/3W
Assuming you are working with the same resistance of 10 Ohms, the 3.7V battery with no voltage conversion will give you a current draw of 370mA and a power output of ~1.4W and in theory with a 4200mAH battery, you should get 4200/370 = 11 hours.Given that Li-Ion have a pretty steep drop off, you get in reality maybe 80% of the theoretical (guess) so 9 hours and maybe a 10-15% drop in performance for cold so roughly 7.7hours.
Now given that 1.4W is roughly 4x the middle setting of the Thermics, you will probably want to go the buck converter route and step down the voltage to let's say 2'ish volts. Following the same logic (assuming a 10 ohm load) you get
2/10 = 200mA
(2)^2/10 = .4W
[(4200 x 0.8)/200] * .85 = 14.28 hours.
Note that this doesn't take into account the efficiency of the buck converter itself, which could be as little as 40% and as high as 80%. Say we split it down the middle and call it 60% you still get about 8.5 hours.Now this is still a pretty simplistic modeling of the system, and even if I am off by say another 20%, that should still get you to somewhere close to 6.5 - 7 hours (and I don't guarantee any of the above will actually happen in real life ;) )
I think it will still take some trial and error, but ohm's law and a few assumptions should get you to the ballpark. Start with the the three basic elements, the battery, the heater element, and some means to control the voltage output. My caveat is ensure you have the proper charger for whatever battery you choose. A non-contact (laser) thermometer may be a good idea to to check the output temperature before putting your feet on it.