I have a box that's nearly the exact same size. Mines is, 15W X 19D X 35H. In my experience, you'll always be combating heat. By the time you reach flowering, you're going to have problems. There's ways to make the heat work in your favor (Like dealing with humidity), but you're gonna need some very good circulation in order to keep the temp down. I've given up on using the box for my whole grow, instead it's now a vegging box. Like somebody said before, exhaust is way more important than getting fresh air in.
My advice:
Place a small 6" fan somewhere in the middle of the top half of your box and angle the fan towards the light. The cool air from the bottom of the box will get sucked through the fan and pass across the light, cooling the hot air around the light and sending it directly to your exhaust.
You don't, or shouldn't, place this fan directly in the center, it should be placed in the corner so that the air circles around the box and light. Your heating problem should greatly improve.
I live in a place that's very hot (an island). Heat is constantly my enemy. I've started to learn how to manipulate it and make it work in my favor. I've also learned a lot on how to combat this persistent enemy. With my box being almost identical to yours, give my solution a try.
