Right now I'm building a 7x4x7.5' box using 1x2's and 2x2's to frame it and I'm going to use to use a staple gun to put panda film around the frame making a sealed area inside for now.

I'm running a 250 HPS now and I'll put my 1000w HPS in when all my girls 8" or so since I have both lights already. I would do a 600w HPS if I was to start over with no equipment. A 600 HPS in a 4x4x8' tent would do nice. My next move up would be to buy another 600 and run both air cooled in a 4x8x8' tent.

You said that it was in an attic with fluctuating temps right?

A) Lung room. You can use air ducting and a fan to suck air from a room with temps at 78° to fill your tent with fresh air.

B) Cool/Heat air. Use a small AC unit to keep temps right nearby your intake.

C) Find another way to stabilize temps

D) Search the board for "temperature" and see how people handle it.

I'll try to give you some tips on making a box once I finish mine.