So, the area is a 2' x 2' enclosed closet? If so, a 1000w HPS is going to have to be super-ventilated in order to keep temperatures down. Will you be able to make some holes in the drywall/attic/door to allow for active ventilation?

Going with a 600w system would help keep the heat down, but even it would need to be sufficiently ventilated to keep from cooking everything in the closet like an oven.