There is some stuff called "Block-IR" that is made. They claim a "97% reduction in thermal imaging reduction." Might be total bullshit, but they had pictures and lol, they say that it was made for NASA. lol
Now I wanted to ask you. Okay, so you have 1 - 600w HPS? Right. Don't even sweat it. If you have awesome airflow, which you should, it shouldn't get that hot. 6" fan on a 600w hps is bad ass. Just make sure you are venting that shit strait out of the room. You really are not going to leave a heat signature.
Second. Depending on where you live they might not even have access to the imaging equipment. Why would they look at your house? From what I understand weed is low enough priority that your small grow would not be worth it to even investigate. Think about the heat. It is not much hotter than some asshole that left his 150w incandescent attic light on for a few days. (might be wrong)
If you have trees it will make it even harder for them to see in. If you are really spooked about it you should make sure that the top of your grow room is not the roof. You should have an air gap. Heat will levelize with the air around it. Fiberglass is your friend. Line the outside of the room with is.
My attic in Texas stays around 90 to 100 in the summer (day time). My grow room would sure as fuck be cooled to 75 to 78. It would show up as a cool spot.