Are we talking about the actual temperatures in your garage? I've never run an MDD in those extremes, but here are some things that you can do to help with cooling...
1. If it has never been done on this machine, apply some fresh thermal compound to the CPU. If you've never done this before, use the kind that does not conduct electricity. I like Arctic Silver MX-4.
2. Install
CHUD and enable nap mode (it's best to create a script to do this automatically each time the computer is booted). This will make a HUGE difference.
3. Additional fans if needed. I have one extra 80mm fan between the CPU and the back of the case to vent hot air from both the CPU and GPU. IMHO, this is better than slot coolers which work against the intended airflow of the machine. It's possible to mount two 60mm fans in this space and two more in the front behind the nostrils. Before getting carried away with extra fans, download
Temperature Monitor to see what is actually needed. 62C is considered the high end of normal operating range. I keep mine clocked and fanned so that it never gets above 50C.
I don't know what to say about the other side of your temperature extremes. I'd be worried about condensation if going back and fourth between subfreezing and operating temperatures. I suppose humidity plays a role there, but I really wouldn't know what to expect. Even water droplets that form through condensation can eventually
self-ionize to the point that they conduct electricity.