Just thought I'd throw this out there...
When I did the reinstall on my Koker, I realized I'd need two dampers; one to keep the coal heat from blowing down into the propane furnace and the other to keep the propane heat from blowing into the coal furnace. I made the first out of a flat piece of sheetmetal. The second I got out of my existing ductwork. The installers put dampers in the main ducts on both sides of the propane furnace to allow me to shutoff the heat to one side of the house or the other
I took one of those dampers and moved it into the duct I was installing.
Maybe you have an existing damper you're not using?