It wont get too hot, but still in a small space it will warm up. In a confined space such as yours your going to need an exhaust fan, and a means of getting fresh air into the grow box. Plus a fan wouldnt hurt.
Dude your missing the point, its going to create heat NO MATTER WHAT. All a fan is going to do is blow hot air around the grow box. You NEED and exhaust in a small space like that, and you NEED a way to get fresh air into your space as well. Theres no other advice i can give you, thats exactly what you need to do.