There is a Corvette restoration shop with available parts near me. These are cloth 4" tubes that fit early Corvettes and fill the restoration requirements for them. However, they are too short for our use, so I had to buy three. On the passenger side it was about 1' short, just perfect for the seam to fit under the Bracket. I used a 4" drier vent, cut to three or four inches and fit (slid) the one whole one and the one cut one, then covered the seam with cloth medical bandage tape sprayed flat black. I glued the ends of the tape so it would not come off.
On the Drivers side, the vent tube was about 1 1/2' too short, so you can see the seam just below the bracket. I don't think anyone will notice, would you?
I used JB Welded T-nuts for the bracket screws. I saw no sense to having nuts that could only to accessed when the doors were off, if I needed to take the vent brackets off. My doors are still off and the heater is out until the upholsterer finishes the top, trunk, rugs and inside of the cars. He already did the dash, seats and door panels.