Electrical – How warm can power cables get while still being Ok

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.

Yes, its still valid.

Power and data cable coatings are insulation - designed to protect from electric shock and to avoid short circuits. They do not interfere with magnetic fields.

I do not know what kind of shielding you are talking about - I have never seen any modern power or data cables with shielding that blocks magnetic fields. Magnetic fields are quite pervasive.

In response to your follow up question:

The size and strength of a magnetic field is relative to the amount of current flowing through the wire within. For very low voltage wires, like network cables, bundling them together generally doesn't create a strong enough field to disturb the other lines in the bundle.

However, if you loop the cables, you end up running the same current through very nearly the same space - over, and over, and over - and this can amplify the magnetic field that's created, enough that it can interfere with other signals.

With power cables, they run much more current and so create much bigger fields. Big and strong enough to interfere with the tiny currents going thru the other cables. Looping them is even stronger and more problematic.