i wouldn't be able to get a stronger fan for output so what about putting some holes in and using the one i have for output alone? And is it REALLy necessary to have a fan inside to direct air flow?
Hi. Using what you have, it is important to remember that when you move your plant to the flowering stage you must be able to keep the inside of the box in near total darkness for the dark periods. These dark periods should be uninterrupted. The reason I say this is because you mention cutting holes in the box. So, if you do cut holes, be sure that they are positioned so as to not allow light into your enclosure.
You will need to mount the fan that you have in a way that it sucks air out of the box. Or, possibly, if this is the only fan you will have, you might be able to rig it up to suck air in and also to blow on the plant to circulate air. If you have this fan mounted so that it sucks air in, then the exhausting hole should force air out. Never tried it, but theoretically, it could work, I guess.
Odor control may be an issue. If you are on a strict budget with no plan to spend anything else on your enclosure, I don't know what you will do about odor control. NL does not create huge odor problems, but...