I'm amazed not more problems than heat. Where is the air escaping from...
1.) Let's get terms correct so as not to confuse - it's an inline fan either way, question is whether to use it for intake or exhaust
2.) you need exhaust, you can do fine on passive intake (no fan). The pull from the exhaust fan should bring in as much fresh air as needed.
3.) you need to exhaust out the hot air - that's why the temp is rising
4.) regardless of all that, to
@Final Phase's point, you need to make sure there are no light leaks. So how do you do this if there is some opening at the bottom for intake? I current have a L shaped piece of ductwork in my intake. I used to have a Bud Lt 30 pack box with some screen over the end there. Point is, find some way to let air in without light.
5.) A few rips, even if not a light issue, will likely let odor out (although that will be an issue if you don't have a carbon filter attached to the exhaust anyway). Sounds like the million and first use for duct tape!