I think you should cut a bigger hole, and go for a bigger fan, maybe at least double that size. Mine are 7inchers, and I have 2 of them, but my tub is also something like 54 gal.

Well, considering your in the north and its winter, using that little fan and placing it in the garage should be fine. But if you want that thing to be long term without you altering it anymore then do it right the first time ya know. That's my two cents.

I do think that fan won't be enough though. Seriously though, get a thermometer, let your box run for a while, hours, and then see how hot it is in there. Should give you a good idea of the temperature difference from room temp. to your grow box temp. I can keep my box temp. only 2 degrees warmer than room temperature. :stoned::stoned: