Which material to use for building in a modern built-in fireplace?

[Evolved]

I have a traditional chimney measuring 0.85x0.95m with 5 channels, against which I plan to install a built-in fireplace. It will be one of the models below, all of which have glass on three sides without grills and a guillotine/lift door. I'm listing all models to assist others who are searching for this type of insert fireplace.

Kratki NBC/NBU
Contura i51
Scanspis 5004
Romotop Heat/Cara
MCZ Forma T95
Spartherm Arte 3RL
Brunner Panorama
RAIS Visio 3
Kalfire Heat Pure 90-3
Caminetti Montegrappa TEC3

All of them seem to be made of steel instead of cast iron. Correct me if I'm wrong, but steel is not as heat-resistant as cast iron. The built-ins I've experienced in reality haven't been warm at all after burning. In my previous house, I had a built-in cast iron insert, where you could feel the heat for almost a day after burning. Could it be detrimental to the fireplace and, not to mention the lifting mechanism, to build it into, for example, heat-retaining brick?

Now to the main question. For building in such a fireplace, what material should I use? Brick, lightweight concrete, vermiculite? Does gypsum work as well? I want to achieve a concrete-colored plastered surface that builds out minimally. And, like in my previous house, be able to burn for hours without it becoming too warm to be in the room.

Thanks in advance!

 

[Dan_Johansson]

Skamotec insulation board at 50 mm is probably the thinnest one can get away with.

 

[Sir Duke]

I have a traditional chimney 0.85x0.95m with 5 channels where I will install a built-in fireplace. It will be one of the models below, all of which have glass on three sides without grilles and a guillotine/lift door. I am gathering all models to make it easier for others who are looking for just such an insert fireplace.

Kratki NBC/NBU
Contura i51
Scanspis 5004
Romotop Heat/Cara
MCZ Forma T95
Spartherm Arte 3RL
Brunner Panorama
RAIS Visio 3
Kalfire Heat Pure 90-3
Caminetti Montegrappa TEC3

All seem to be made of steel instead of cast iron. Correct me if I'm wrong but steel is not as resistant to heat as cast iron. The built-in models I have felt in real life have not been warm at all after burning. In my previous house, I had a built-in cast iron insert, where you could feel the heat for almost a day after burning. Could it be disastrous for the fireplace and not to mention the lift mechanism, to build it into heat-retaining brick, for example?

So, to the main question. To build in such a fireplace, what material should I use? Brick, lightweight concrete, vermiculite? Does gypsum board work as well? I want a concrete-colored plastered surface that minimally protrudes. And just like in my previous house, be able to burn for hours without it getting too hot to be in the room.

Thanks in advance!

Hi!
How did you solve it in the end?
I am looking for something like minerit but with the same dimensions as gypsum boards... But can't find anything...

 

[Keviin]

Which record/block did you use?

 

[Rasmuzs]

Yes, how did you do it?

 

[Sir Duke]

Yes, how did you do it?

I used Cembrit fiber cement board. Same dimensions as standard drywall. Flush with the drywall walls next to it.

It is fireproof. So no external insulation was needed.

I painted everything with regular wall paint afterward, and it has withstood the heat so far.