Add mass to fireplace insert

Hello everyone

did any of you already tried to "add mass" to a fireplace insert ?
Those things burn incompletely and they get cold very quick, I would like to add mass.
Attached pics of the insert.

Any idea guys ?

Thanks a lot

Those look more like prefab fireplaces than inserts but it's hard to say.  If they are prefabs there really isn't anything that can be done.  Regardless adding mass will mean it cools down slower but it will also heat up much slower so there isn't really any gain.

The incomplete combustion is probably due to wood that isn't dry enough