My problem is that the heater is located at the rear of our van, transversely, so the ducting has to take a tight turn for it to run forward. I need to box the heater ducting in also, so any space I can save is welcome

can I ask please which heater make and model you are installing? It makes a difference on the best way of achieving a 90 degree bend in the heater ducting. It sounds like we can do this very close to the heater, yes?