A duct can be made, it just takes time and some thought. I would make a temporary duct out of cardboard. Just to see if it will work the way you want it to. Then the carboard can be used as templates for the metal duct.
The big thing about making ducts is getting a cold air return to the stove that then is heated and sent out to the house via the hot air duct.
You might try seeing if a heating contractor could come over and give you some ideas. I don't mean just get a quote and never call back. Offer to pay for an hour of time for the house call. Make sure you get a duct-work guy to come over, not a furnace salesman.
Make a few phone calls, you might get someone with experience doing just what you have in mind. Ductwork is easly once you have a little experience and the right tools.
Otherwise just try some tape and cardboard to start and see if it helps get more heat upstairs. Or a floor register in a strategic spot may do what you want??
Hope this helps. Greg L