If the air you are drawing in is 73F, you're probably not going to do much better than having the temp in your box at 85F. Using your A/c would help if you tapped into it. If decide to do that, just cut a 6" round hole into it, and see how much air will come out for your to use. If its in the middle of a large duct, and you find you want to get more air out, then you'll have to make an adjustable damper and place it just past the hole you cut in. Here's what they look like, and you can make one yourself --
BROAN MFG CO INC (NAUTILUS) BP87 DUCT DAMPER
It's basically just a piece of metal to re-direct air where you want it, so instead of Remember though, it only works when your thermostat is calling for more cool air where your thermostat is located and doesn't care what the temp is inside your box. So it will only blow cold air when the house is needing a/c not necessairly when your box wants it.

My opinion, it's not worth the hassle as long as your temps don't go over 85.
SpaceNeedle Reviewed by SpaceNeedle on . Manging temperature in a 4x4x8 box Hi all. I have a 4w x 4h x 8L box and i am having a hard time getting my temperatures down to the high 70's. Running 2 x 400HPS They were over 100f and I have installed a 375CFM fan with a carbon sock and the temps are now running about 85f. There are 4 small 4" fans inside the box and two 4"x6" opening cut into the bottom as passive intake. Rating: 5