Is this a custom AC duct?  

Wow, I didn’t know I could be an HVAC installer.

It looks like nitrile butadiene rubber which is an appropriate treatment for ductwork. Any post construction installation is going to look something like this. I wouldn't be surprised if they hired someone to do it.

Damn, the duct came insulated. Next you will say it needs to be sheet metal, but then complain it ISN'T insulated and they need to add foam. smdh.

HVAC Tech: That will be $35K to replace 4 feet of duct

Does it work?

ain't pretty, but does it work?

LUL