In the video mentioned above, the narrator talks about putting some inserts in to fix some oversize bolt holes (previously stripped out). It appears to me that those inserts would just pull out when the screws are tightened to hold the cover up. I think they need to be on the other side to work as designed..
Sealing up ducts is good practice. So far, our 13.5 unit keeps up fine, but have not been in the direct sun with temps above 100 this year. Last year it was in the mid to high 90s and cooled fine.