Amazing man it really looks so good. Love the big window and placement of the door more towards the front makes it look like a big camper. Makes me long for making one myself! I find your pictures move a bit too fast as I don't read that fast and can't view the picture fully.
I used a 3” pvc drainage pipe that I just cut to length. Inside of it I just put a small L-bracket with a rivet nut in the center. The blue end cap seals it well but also put a machine screw through that blue cap & rivet nut. On the ducting hose I clamped an 90 degree pvc pipe that is friction fit to the pipe in the camper. It’s all cheap and DIY.