Thermostat-controlled corn heater sized for a shop/garage

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.

Rooms above garages are often done improperly. It's amazing, because it really isn't all that difficult, but a lot of people seem to mess it up.

Unfortunately, there's no "easy" way: You're going to have to remove some drywall to inspect and see how the insulation in the ceiling is done. There may or may not be a gap in the space, depending on how the ceiling was done. Even just taking the temperature above the insulation in that space will tell you a lot (above the insulation, it should be close to room temperature).

Drafts in that space are absolutely killer, it means there is a direct source of outside air. Black on the insulation is a good sign of airflow, indicating drafts.

Basically, if the ceiling space isn't totally sealed, the only thing you can do is turn the garage into a conditioned space (very expensive, in terms of both construction costs and ongoing energy costs), and even then, if the problem is between the insulation and sub-floor, it won't help.

What should be there at a minimum is a continuous vapour barrier, with a layer of insulation on top. The vapour barrier should be sealed to the subfloor or the vapour barrier from the walls above. The insulation should go all the way around the edge (in the headers), so the insulation is continuous from the floors up to the subfloor. There should absolutely be no drafts, exposed concrete, or anything directly connected to the outside that is uninsulated.

The best way (in my opinion) to seal this space is to use closed-cell spray foam, and form a continuous seal across the bottom of the entire subfloor. This gives you great insulation, no drafts, and acts as a vapour barrier as well (vapour barrier is not needed in this case -- though some building inspectors still don't understand this, so check your local codes). It also is better at ensuring fumes from vehicles in the garage can't get into your living space (in theory, vapour barrier prevents this as well, but 6mil vapour barrier is easier to puncture than a couple inches of hardened foam).

Though you may be able to find something obvious and fix it, be prepared that there's a possibility that the only "fix" is going to be to completely tear down the ceiling of the garage and re-do it properly.

If you're going to spend money and time on this, do it right.