I have an attic grow and I am in the South TX area where it gets very hot. I built a 4' x 4' x 6.5' tall grow room and was very worried about temps but I have been able to keep them in check.
I would suggest using some 1" rigid foam (R5) with foil backing on one side (keep light out) around some basic 2' x 2' or 2' x 3' wood framing for walls. The rigid foam boards are available at any home center for about $10 per 4' x 8' sheet and are easy to cut to size with a utility knife. I then added some R13 insulation around the room for a total of R18. I am feeding the room off the main house via a 6" duct and venting out an attic ridge vent. I have one 4" 70cfm fan for the room and one 4" 170cfm fan to vent my 600w light with an air cooled hood. And the room was sealed (seams) with foil tape.
About two weeks ago we had temps over 105F and my attic was 147F!! Grow room only hit 86F, it normally stays around 80F and drops to 74F (upstairs a/c temp) at night. The biggest pain with an attic grow for me is when I have to tend to the plants during the day - I have get in the room quickly and close the door behind me or the temps sky rocket in 20-30 seconds.
Anyway - hope this helps with some ideas for you. Pic attached..