You don't push the air in, so you don't need an intake fan. You need to exhaust it, push it out, this way the negative pressure created in the box will ensure that just the right of air will go in, equalling the volume exhausted.
If you push your air in with an intake fan, and your box is not 100% airtight, you will have problems with odour control later on, as the skunky smell will be escaping from any crevice it can find due to positive air pressure build-up, and not going through your air filter located on the exhaust.
So, again, no need for an intake fan, this is an expense you can spare yourself from. Just drill a wide hole that is 25% larger in diameter than your exhaust and put a light trap over it.