Betcha you could toss one simple regular sized (5000 btu) window AC unit up in there at whatever end the air flow comes from the most and blow it on high in that same direction and never have an issue. Just a thought. It is pretty easy and maintenance free, and in your situation you wouldn't even have to seal anything. Just put it in and let it run on the coldest setting it's got. Personally I'd go with an 8000 btu unit by Toshiba that has digital readout and bluetooth controller, also ability to set the air temp to anywhere from 64 to 78 and keep it that way, and also has a dehumidifier function that will significantly reduce the humidity when needed although it doesn't cool as much on that setting. WELL worth the $315 at Home Depot in my opinion. Just my two cents thoughts. I'm sure you've considered it or something like it. I'm here to tell you it works really really well. You can look at the square footage you're trying to cool and compare to the projected square footage the box says it will cool and figure out the best size if you went that route. Besides temp control, they REALLY help with humidity - I've been keeping the photo tent at >45% for budding, which is not easy.I would have to find a taller attic. The middle ceiling in there is 6'6"... most tents are 7'. I gave away several of my tents and decided to make this work. Have I told you about the indoor rain I can produce in the winter time by heating the attic, bringing in moisture, and letting that work against the super cold metal roof just inches away? It was ugly. Without extensive insulation installation, winter grows are not going to be possible in that space. Summer heat is much easier to deal with (no inside rain) and all I need to do is set the house temp to 70 and then fan that cold air into the rooms. Without, it can easily get to 110 in there... with proper airflow management, I can keep it below 85.
Something to consider perhaps!