I don't think there's anything inherently about the Maglite that is causing the batteries to fail. A polymer light will also get hot or cold, just maybe more slowly. But it will freeze or boil just the same as a metal light.

It's really the battery and the "wrapping" inside it that is breaking down and allowing the chemicals to leak. How long were those AA's in your light? I never let the alkalines in my car lights to go for more than a year before changing them out and I've never had any cells leak. And by "change" I mean using replacements that haven't also been stored in the car that whole time.

For a AA Mini-mag that you think you may need to put into service on a moment's notice and you really want to avoid any more leaking batteries, then it's probably best to just bite the bullet and put a couple of lithium cells in there. For replacements, it's fine to have a stash of alkalines that you burn up during some actual situation. Just keep them in a container or plastic bag that will catch any spills if one of the cells leaks in storage. If you're not that concerned about being able to put into service right away, there's nothing wrong with storing the batteries separate from the light, too.

Another option not mentioned yet is to use NiMH rechargeable AA's. Best would be a good quality, low self-discharge kind. I don't think I've heard a single report of them leaking, and the new ones can hold a good charge for a long time in contrast to the older NiMH cells that would be dead in a couple weeks. Of course, there's a higher upfront cost to buying the cells and charger compared to buying some lithiums, but it does give you the option of being able to recharge them during some situation, for example, using your car's accessory jack or maybe a small solar panel. Might come in handy in case you aren't able to find a way to buy more batteries later on.