I can't say how hot it really gets. My room is inside the house which is fairly climate controlled. I've seen lots of people mention that the ballast goes outside in smaller rooms as it adds to the ambient heat. Now if it's winter and your growing in the garage, you might want that sucker inside your room/box/whatever. I think size and ventilation are the two determining factors.

I like mine outside the room becuase I'm a sloppy waterer. Less chance of splashover and shorting out my ballast. Plus, more room for plants.

I got the panda plastic at a local hydroponic store. It's like 3mil plastic, black on one side and white on the other.

My room is quite portable and easy to disassemble for cleaning/moving. Still working in new ideas to come up with a final plan.

Just stick a thermometer in there to get some basic readings before you put your plants in, then you can test. My room varies between like 75-90, depending on what i have running.