Well, i was saying that you should insulate the outside part.
Solution 1 you could plug the fireplace with bricks, temporarily, untill you
sell, if you ever do. Or air tighten the glass on the front of the chimney, so you don't have
CO2 comming back in, but you could leave that front unchanged.
Or use it to fit a batch rocket firebox, which is what i would do. Best of both worlds. Eficiency, and looking at the fire. Well, looking at this, if it was my house, the batch rocket conversion would be already done by now!
https://lh6.googleusercontent.com/Tk-HCwFSqQWO9LF6k3Pxvq7tiwQPX6mNgUpa904NHXVk2v6PnCb6P-FGWc3KneELzg
You don't have much to insulate either.
https://lh4.googleusercontent.com/rLJxYM-FWp5yPObNdGpJG7KMqI7-aV3F_EAXVj2-Ol5AhkMzQdELSE50Z0ywfN8piw=s640
Well, my opinion. After seeing the pics, because i replied too fast. Forget about your actual stove pipe. Put a plunger tube in the chimney, i bet you have plenty of space in there. Concrete around it, at eave's height, just a little 10 or 15cm slab. That would be the top of your bell. Stuff some rockwool above. Let the plunger tube go above the top of the chimney, you put a piece oo steel above your chimney for waterproofing it. and let the plunger tube through obviously. Remove the doors, the frame if you can. May be the top of teh firebox (there's usualy a
cast iron piece. )
Stuff a firebox similar to this one in build.
http://donkey32.proboards.com/post/11503/thread
A bit bigger, this one is only a 5 incher. With a 4 incher firebox. Then insulate it with rockwool or
superwool, held by chickenwire.
Redo a refractory brick front, around the mouth of the firebox, to replace the glass. You could even do it in steel, screwed to the frame. That would radiate a bit of quick heat in the room.
It's exactly the same principle as this one. Open this sketchup file, and orbit, or hide components, you'll understand exactly what i mean.
https://permies.com/t/31382/a/13956/chimneybell2.skp?download_attachment=true
And then, on the outside, you pile strawbales against your chimney, up to the eave. Then atach few battens, and do a bit of wood panneling the same as you have both sides. With the materials, it wouldn't take me the day i bet.
You nearly have all what's needed. You don't even need to drill holes.