Fire Extinguisher In Unheated Garage & Shed

I used to be a fire extinguisher repair tech. This unit should not be refilled even though it is possible with the right equipment. The plastic/nylon heads will develop cracks and leaks over time. Buy a new unit with a metal head. Those can be refilled and will last you a long time. Amerex, Badger, General, Ansul are all decent brands.

First off, as long as the temps stay in range, I don't think you'll have a problem with an 'air-tight' container. The only reason computer equipment needs airflow is because it's dumping heat into that air. As long as you have a little bit of air, and can keep it at an acceptable temp, low airflow won't be a problem. You can't go zero airflow because the equipment in question isn't built to heatsink to it's case.

Off had the first thing that occurs to me is a 'garage fridge'(something like this: http://www.amazon.com/dp/B001B2P2G8/?tag=googhydr-20&hvadid=3788940351&ref=pd_sl_6z7wf02xh3_b)

The thing includes a heater system to keep it's refrigerated section from freezing in cold temps.

That's really more than you need though, because it's going to try to keep stuff cold when you really only need it to keep stuff room temp. It's going for a much narrower temp range than you'll care about.

If I were going to try this, I'd get an old fridge (doesn't have to work), and build my own controls (some kind of micro-controller) - I'd cut some kind of vents with controllable louvers and fans into the top and side. And some kind of heating system (really small space header? dunno). I'd add in a temp sensor, and turn the heater on when things get too cold, and the fans on (with louvers open) when things get too hot. If the temp gets out of range, cut power to the device in question.

If you want it to deal with hot days as well, then get a fridge that WORKS, and put the chillers it into the mix as well - if things get hot enough that the fans aren't doing it, cut them, close the louvers and hit the chillers.