I purchased some silicone joiners which I have used for connecting the air vent fexible ducts. I used 2.5" straight join on the cabin air vent and a 2.75" to 2.5" reducer on the NACA inlet. The 2.5" straight fits snugly inside of both the fexible duct and the cabin air vent outet. The 2.75" end fits over the NACA inlet and the 2.5" fits inside the fexible ducting.