I have this cover by Formosa for my TJ and will let you guys know that its the best cover I have come across. My TJ sits outside and not a drop of water gets in. I would buy it again if I needed one for the JKU.
https://formosacovers.com/collections/vehicle-automotive-jeep-covers
Here a good write up on blend door issue
How to Repair Blend Box Door on a Jeep Wrangler JK | Jeep Wrangler JK Forum
https://wranglerjkforum.net/threads/how-to-repair-blend-box-door-on-a-jeep-wrangler-jk.26103/
I used a windex spray bottle and it worked, it was winter when I applied it. I ran the spray bottle under hot water to make the FF more liquidly. Maybe using a spray bottle running of an air compressor might be better.