I don't think you can use ductboard if it's going to be exposed to weather. Duct board is made of fiber glass, and it will probably breakdown from sunlite and rain. I install ductwork for a living and can tell you that it is not a quality product. Yea you can cut it and make it your self, but the airflow isn't as good, and time will tell how long the stuffs going to last until breakdown occurs. I myself don't want to be breathing fiberglass particles.
Why don't you want to install galvanized metal? If its a cost thing, go to a construction site and see if they have any ductwork they are pitching. They would be happy to give it to you. Get yourself some tech screws , a grinder and some duct seal and make yourself an intake. If you have the capability to make a stove out of hot water tanks, surely you can make some duct. Also try any of the HVAC companys in the area. they always have stuff that they demoed laying around. Chances are you can get yourself a good used blower to put in there.
Flex duct wouldn;t be a good choice either. Same thing. Durability airflow problems. I know they use miles of it in residential construction but thats cause its cheep to install, not because its good. With the cost of energy being what it is these days, it really pays to have a good working system. Whats the sence of having a 90% effecient furnace if the airflow isn't good.