I've only mentioned it 3 or 4x.
Reason you MUST/NEED/absolutely with out a doubt need a fan is for this reason.
A portable a.c. system uses a fan to expel the exhaust out of what 6' max oem hose. A window unit doesn't really have a fan that expels the hot exhaust air out since its ment to be outside not the way we have it set up. A window unit uses a radiator and fan of sorts luke in a vehicle. Except an ac pushes air thru the condenser and out of the unit, vs a car sucking air in over the radiator. So, the window unit needs help exhausting the air since your choking off the open/free exhaust air it's supposed to have.
I'm starting over from.scratch now. New 8" r6 ducting, 8" 720cfm fan, and new huge box. Or I may say F it!, and hust leave my portable setup as is and go thru gas a bit quicker. Been rocking it out last 5 yrs like this.
Just trying to help, since I've been where you are. Bandaids don't work, overkill it and never worry about it again. Same reason I've never had any duct work fall. I use hvac tape and worm clamps.
every one keeps telling me to do this and it will fix it and yet nothing as first I was told I need one ex fan then 2 then some box fans then people say o u get a 5k ac and that will do it so far every one keeps saying though all my money at it and maybe something I buy will help
Edit
All you'll need is a 4" HO fan. At least 250cfm. Plug that into a 3 way (that can handle the amp/watts) with the ac and it will only cone on with the ac.
If you were local, I'd rebuild it for you In 1hr or less. Just medicate me as my back should be hurting afterwards. Lol
Mine will be a 2 day x 8hrs to finish.