"what fixed his was to obstruct the top 1/3 of the fireplace opening with a piece of sheet metal-essentially blocking off the top third of the opening into the room"
I had a comercail wood stove that had a sheet metal flap built into it like that. When I opened the door to load wood the flap held in the smoke to the top half of the burner, when the flap would get stuck raised for a few minutes the smoke would roll into the room. I had a bad draft the way I ran the pipe. I don't know if that would actually help with a down draft much though. If not much of a down draft and seldom it might work. The down draft is more like fan forced air, not just a lack of ability to rise.
Shouldn't hurt to try it if you have a piece of metal laying around.
My problem was I came off the stove with a 90, went 2' or so sideways, then 90 back up again to get out the wall through an old airconditionare shell. Not a good draft that way, but worked ok while I needed it.
I have alot of turbulent air here right next to my stove pipe because of a very big tree outside really close to it. A cap on top fixed my down draft problems there.
Only place I can put the wood stove inthe house, and I ain't about to cut down that tree either