You can go to a fireplace shop and get some ceramic fiber insulation they use to line chimneys. A brand name is Kaowool. It is very heat resistant and you just stuff some of it into the opening. Problem solved.

Look into cultured stone

@Annie: Thank you for the link, it's from a reliable source so I am considering carefully what it says. Both of our stoves are connected to outside air and have been for decades. The inlets are on the typically upwind side of the house. Except of course, when we're having a Nor'easter. This house was built before the Civil War and has no insulation in its wall cavities (and none is possible due to the contruction of the wall assembly). So it defines a "leaky house". No problems here with "sick-building" issues related to being too tightly sealed. Sometimes I think we might as well be living outside! When we installed the O/A inlets it made an immediate difference in the draftiness of the rooms so I would be reluctant to go back, but fire safety is a paramount concern for us so I will be talking to my husband about this. We have no other combustion stacks (obviously no furnace or boiler, and our DHW is electric) so we have no carbon monoxide risks with or withhut the OCA. But I am planning on installing powerful vent fan in the current kitchen which is where the wood-burning stove is. (I'm moving the kitchen to another room to get away from the woodstove, but it will still close enough to be affected by the fan's power, I think.) I'm picturing in my mind the pathway of the combustion air within our stove and trying to decide if I have a risk of drawing super-heated air or coals back down into the combustion inlet piping (which is utterly unshielded along its entire length as it lies slap up against the wood sheathing of the wood room) if the system was depressurized, and I think not. But this is particular to my stove, and model, and not true for many other stoves that would simply feed the air directly into the firebox. I really appreciate the heads up and will ponder it some more, especially as we are slowly getting around to installing the new stove (which likely has an intake air pathway that is completely different.) @mjlb: LOL at your DH's association to arched doors and a crematorium. Fortunately the woodburning stoves like the Jotul, while perhaps slightly arched on top have lovely glass doors that don't seem like crematoria, at least to me. Soapstone (or other masonry) thermal mass stoves are fab, but not really the same fire-appreciation sense as any fire place or regular woodstove. They are intended to burn much smaller, hotter fires, often only a couple of times per day. They also are not really retrofitable into an existing hearth and chimney stack since they require different stack pathways and massing. Plus they are extremely heavy and must have a substantial foundation. I love them though, and am considering converting from woodstoves to masonry as we age in place. Woodstoves require such constant vigilance for safety and a considerable amount of physical activity to attend (even leaving aside processing the fuel from tree to firewood) that I can see it may be a problem at some point. We're in our 60's now. With no other source of heat if we couldn't burn wood we'd have to leave the house. My late MIL in the last year or two before she died had issues around her wood heating and cooking arrangements. Her cognition was becoming muzzy enough that I had decided she couldn't live alone there another heating season. Plus as she became frailer just loading the logs into her stove started to become a problem. I remember one miserable day when I split her entire winter's wood down into smaller pieces because she was forbidden to pick up anything more than 2 lbs. She wept the whole time I did it, and I felt so sad for her. A masonry stove since it is fired only two, or at most three times a day could be managed by someone paid to come in and attend to it. The rest of the day it just radiates heat, mildly enough so that you can sit on it safely. I considered getting a soapstone-walled woodstove before choosing the cast-iron Jotul. The soapstone walls are NOT safer to touch than cast-iron (unlike a masonry heater made of soapstone which relies on thermal mass). And having looked at several in the shop being repaired (cracking) we decided to stick with the familar cast-iron. There is one nagging issue for me around wood-burning: it can contribute to diminshed air quality from particulates in the smoke. When you get a whiff of a wood-burn, it usually smells quite pleasant, but in truth you are inhaling particulates and smoke, even in the cleanest, most efficient burning ones. One wood-stove is a trivial air degradation, but wood-burners in every household is a community problem. In many parts of the UK (and some cities here in the US) wood-burning, not to mention coal which is even worse air-quality-wise, is illegal. Stilll this is a harsh climate and some form of heat is necessary. As we have the woods (and generally only use thinnings and blow-downs), and process it on the farm, it seems environmentally better than oil or worse, fracked gas. But it is not totally environmentally benign. We have installed more solar panels to make power, and will add more to provide some electric heating capacity as well. There are no free lunches when it comes to heating in a cold climate. L.

Try hearth.com forum. There is probably people on those forums who can help you.