I've had the 400w hps light from htg for about 2 months and i'm using it in about the same set up. It's in a closet with sliding doors. i keep each side open about 4 inches. I use two fans, on on the floor, on one end of the closet, blowing up towards the light, but also serves to keep air moving over the plants. the other fan is in the top of the closet on the other side and serves to vent warm air out. I also keep the ballast outside of the closet. But i've never had temps at the canopy top 82 degrees.
The light works great, but i wish i had put the money into an air cooled hood at the time. it would save me some money on my next project of building a grow cabinet or modifying the closet so that it'll be light proof.