We modified the metal plate taken from the damper of the old metal heatilater that we had previously removed. It took some grinding and reshaping to get it to fit the new channel which will basically create the throat of the fireplace and is now made with firebrick instead of steel.
In this case the damper plate is modified by having a rod welded the length of it (slightly off centre), so that it is heavier on one side and so tends to want to tip. This off balance aspect is important .
Both ends of the rod will be imbedded into the side walls enabling it to pivot and flop down either way. The most difficult thing now is to position it (in a properly formed brick channel ) so that it is able to swivel to a position where it can stay open (allowing smoke to pass both above and below the damper plate ) while also being able to pivot back the other way to stay closed, simply because the plate's axis is off centre The damper has to rest snugly in both positions without getting caught on the side walls as it is rotated.
The photo above show the damper at about 45 degrees in the full open position.
All this fitting and damper work has to be completed before setting the stones in front over the arch opening. There will still be a lot of reaching up inside to finish the brickwork connecting the visible stonework of the arch up into the throat reaching up to the smoke shelf. Which I haven't talked about yet.