Styrofoam building insulation (Score 3, Informative)

I've done stuff like this with computers for balloon payloads that go up in the stratosphere, where it's around -50 C. Here's two tricks that should help. Trick number one is build a box out of styrofoam building insulation and duct tape. Assuming you're in the US, you'll see a number printed on the insulation like "R5" or "R10." That's the thermal resistance, in BTUs, hours, deg F, and square feet. No, I'm not making that up. Guesstimate the power dissipation of the computer and use that to make the first design, then test and iterate. You'll want to stick a thermometer on the case or other convenient location. If this isn't reliable enough, then trick number two is design your insulated box to run a bit cold, and build a thermostatically controlled heater. We usually designed our own, because we like to do things the hard way, but I believe at someplace like Newark Electronics you can buy a little package that contains a heater and a bimetallic thermostat, you just supply the power.