Even though that 1000 w hps should kinda act as a heater during the day, putting a space heater in there and experimenting with it a little bit until you get to the temp you want is probably the best way.

Your gotta be careful where you plug it in though, Space heaters use alot of electricity. I would use a totally different outlet from the 1000w hps. If i use my heater, two lights, and all of my fans all at the same time The power goes off and I have to go reset the breaker.