Replacement left defroster hose (and adapter?) for a non-a/c 74 Duster.

Hi All: This topic has likely been addressed here before, so my apologies. The original cardboard and metal left defroster hose on my 74 Duster is torn and I need to replace it. The right hose is fine. I've seen the expensive plastic flex duct replacement kits, but I don't want to buy all that stuff. My understanding is that the defroster vent in the dash is a 2.5" vent that will accept one of the plastic 2.5" flex hoses. I believe the attachment to the heater box is smaller (2"?), so there will need to be an adapter to adapt the 2.5" hose to the 2" heater box connection. So all I really need is the hose and the adapter. Does this sound right? And if so, is there a cheap source of these two items? How much hose should I buy? Any and all ideas welcome. Thanks!