You do not want to connect (only) unprotected batteries. There is nothing to stop them from draining too low. A resistive heating element would drain them well below a damaging level.
While a 3 cell pack closer aligns to that flexible element's needs, it is not what I would do. With already limited battery capacity to heat, I would not reduce the # of cells. Plus, that particular element is 75W, could get too hot and is roughly 6A current so from a series 2A battery pack, you'd only get 1/3rd of an hour... really longer than that because as the pack voltage drops, so does current but you're still not wanting the pack to drop too low.
Using a 75W @ 12V element, I'd sooner go with a 4 x cell battery holder and rewire it so it used 2 parallel series of two cells. Wattage drops but runtime gets up to a few hours which is a parameter you didn't mention but it seems as though if you're going to bother, it ought to run for a few hours.
The other issue with that element is it's only 1000mm. Usually you would want the heat at your extremities the most and that's not long enough to span from hand, up arm and across chest over to the other hand, nor do your legs. If it were a survival situation and you didn't care about not losing your fingers and toes as much as keeping core temperature up then I'd sooner think wrapping it around your midsection would be acceptable.
Otherwise I'd look at getting 2, or 4 elements setup for 12V and 1/2 or 1/4th the rated wattage each, again run a 2 parallel, 2 series battery pack if not even more cells, OR run the heating elements in series or for 4, 2 parallel series of 2 then your 4 series battery pack would work unaltered.
That's just a shot in the dark, ultimately a lot of testing would be needed to hit the right runtime vs heat. It's not going to be something quick and easy to do without any voltage regulation unless you get very lucky and even then, unprotected cells are a bad idea.
Even though it's an efficiency loss (which becomes heat, so not all bad in this application) I'd want an adjustable voltage buck circuit, then you have direct control over how hot it gets vs runtime, and a custom battery protection board between the pack and it, that can be set to disconnect the pack when it reaches (# of cells * 2.8V/cell = lowest safe) discharge voltage. Even that depends on using roughly equal capacity cells so they're all hitting 2.8V near the same time. Generic 2000mAh cells you might not be able to trust to be that near in capacity without matching them using a charger capable of rating their true capacity, and should retest their capacity after some use.
It would be easier to just pay $160 for the pre-made heated jacket where they've already done the work for you, and where you have someone else to sue if you burst into flames.