Confused Barametric Dampers for Dura Vent Doublewall?

I have a new Harman SF-A 1500 wood/coal furnace that have just installed in my basement. My instruction wanted a barometric damper installed in my chimney to control my draft which I feel is a good thing!
My problem is that the application required double wall vent pipe due to close clearances in and round my LP gas furnace. Installing the furnace went well except I couldn't find a double walled barometric damper. I used 90 elbows instead, just to get it up and running hoping I could figure out how to do a barometric damper later? I really could use some help!

I don't know if it is practical for you, but my oil burner for years had a baro installed directly in the brick chimney, above and at a right angle to where the metal pipe entered the brickwork. It was a heavy-duty affair, moving plate was quite heavy steel or cast iron, but it was well-balanced and it worked just fine.

We need to see more photos of your installation, particularly the smoke pipe from the Harman to the chimney.

Pics of the smoke pipe from start to finish will help...
A double wall Tee will do the trick...
Have a stove shop make an adapter that will fit the Tee end and the baro end...
That is what I did with my double wall install...
Just need to face the baro away from combustables...

From the picture it looks like you have single walled pipe right out of the furnace? If so put the barometric damper right after the first 90'.