I would make your 18 hour photoperiod from 6pm to 12pm. Run the lights when the outdoor temps are lower. Additionally I would install a passive intake in the base of your growbox. Anything will do, you could get a black/brown dryer exhaust port and place it facing inwards (anything that will close when air is not being cycled). Place a fan dedicated to exhaust in the top of the box (The other side from the intake port) and run it solid from 6pm-1pm. Blows air outwards, new air is sucked into the box below and cycles up through. 6pm-1pm cools a bit past when your lights shut down, I'd have it pop on for 15 minutes every 2 hours while they're small and every hour when they're larger. You can fiddle with the exhaust periods based on how cool your box starts to run.
You have 12 square feet, a 50 CFM fan will be plenty, and a 90 wouldn't hurt. Turning your box into a wind tunnel is likely the best way you're going to cool it, just make sure your plants aren't being blown on too hard. 50CFM is replacing the air 4.15 times every 60 seconds. Putting your growbox on lower levels of your home, the basement, etc will help. Anything with a lower ambient temperature will result in cooler temps in your box.
Plants will survive @ 90 degrees with no physical effects although they will grow slower than they would at a cooler temp. The biggest caution I can give you with running warm temps is WATCH YOUR SOIL MOISTURE! Do not overwater, but when your medium goes dry (which it should from time to time to promote roots oxygenating the soil) your roots will cook much faster than other growers. The above ground portion of your plant can withstand very high surface temps, however if the roots start spiking. . . you'll have some MJ potpourri pretty quick.
