Lithium D-cells seem to be rare and expensive.
I suppose he could get AA to D size adapters. Runtime wouldn't be nearly as good as with D-cells, but since this is a car light that clearly isn't needed often, maybe lumen aeternum would be ok with that.
Now as to the question about temperatures: I don't know about fluctuations, but high temperatures definitely do decrease the storage life of alkaline batteries, in addition to reducing the effective output and capacity of the batteries. If the light is stored in a hot car, that will definitely affect how long the batteries can be usefully stored - Energizer rates the storage life at 40 deg C (104F) at a little over half as long as at 20 deg C (68F) for alkalines. Your car can potentially get well over 120 degrees in hot weather, further accelerating the loss of performance.
I'm guessing your batteries must have been getting near the end of their lives when you last checked them, and a few more months was enough to finish them off.
For lithium primaries, Energizer says there is a slow loss of capacity over time at high temperatures, but they still consider the battery to be usable for roughly the same amount of time (over a decade), indicating it can continue to supply a useful current.
Energizer has a very useful comparison of performance versus temperature for the different battery types except lithium ion here. See page 1 and 4:
http://data.energizer.com/PDFs/temperat.pdf