Used one of the elbows from a Mercedes and cut most of the square end off so it would fit inside. It was a bit bigger in diameter, so cut a wedge out to reduce its circumference.
Put a hose clamp around it once it was in the right position. Then using a soldering iron welded the seam back up. Been a while since I had welded some plastic as used to do a bit now and again back in the 80's before YouTube was around to show you how! ;)
Welded all the way around and even inside to make a good strong connection. I used a 80 watt soldering iron as was thick, but normally only use a 40 watt one. Don't want it too hot as leaves it brittle when it starts smoking a lot. Always have a fan going as the fumes are toxic. This gives you an idea of where it sits when screwed in the end of the dash.
Got both sides done now and will use the original ducting from the Mercedes as well which was a bit over 66mm-2.5". It will hook around in front of the defrost vent and go to the front most outlets at the top of the A/C unit.
Before I removed the A/C unit from the cab, I made a template to show me how much room I had to the cowl above and to the firewall behind to run the defrost ducting. There was just no room to run it out the top so I fitted these PVC elbows to the underside of the defrost chamber. I glued a collar tightly above and below where it enters in to hold them in place.