Seems like a similar problem I dealt with when still using my cabinet. I packed a 600 MH/HPS (dig ballast) into a 2 x 4 cabinet. Actually didn't do too bad and learned a bunch. In the cabinet, I used an active intake consisting of 165 cfm fan with 2 4" duct boosters. Exhaust was a couple of bathroom fans.. all controlled with a CAP AIR-3dn. All worked pretty good about 9 months out of the year in the garage. You can see the results in the links below..

In your case, I think those portable A/C units may work pretty good. The downside is the loss of floor space.. Can you go with the A/C unit and keep your 600? That would get you closer to your goal..

Farmer Rich
Farmer Rich Reviewed by Farmer Rich on . Temperature Control One and all, First post, thanks all in advance. Specs - 3'x4' indoor closet, hooded 600W MH vented to roof with squirrel cage fan, box fan in ceiling, mylar all around, 6"x10" floor intake vent, room/closet kept off HVAC system during winter. Cannot use windows. Having tremendous heat problems. No matter what it stays 80-85*. When I use the central HVAC it only drops 5* or so and still found it above 90* in the middle of summer. Ruined a couple of crops already. THC being baked off Rating: 5