It's not that hard. All you need is a lot of mass surrounded by a good insulator. The heat has nowhere to go and leaks out at a rate controlled by how efficient the insulator is. But only at the surface area of the mass which relative to the volume is quite small.
There's a basalt based thermal storage system in the Netherlands that is used for seasonal storage. It's heated up over the summer using solar energy and during the winter they pump water through it for heating. It's Basalt in a metal box surrounded by wool and it stays warm through the winter. The temperatures for this system are much lower (500 degrees) but the principle is about the same. https://materialdistrict.com/article/battery-natural-stone/
So, if you can heat the mass to the desired temperature, keeping it there is relatively easy. It would slowly cool over time but not at a rate that is problematic and only very slowly. A large enough system might store heat for months/years.
There's a basalt based thermal storage system in the Netherlands that is used for seasonal storage. It's heated up over the summer using solar energy and during the winter they pump water through it for heating. It's Basalt in a metal box surrounded by wool and it stays warm through the winter. The temperatures for this system are much lower (500 degrees) but the principle is about the same. https://materialdistrict.com/article/battery-natural-stone/
So, if you can heat the mass to the desired temperature, keeping it there is relatively easy. It would slowly cool over time but not at a rate that is problematic and only very slowly. A large enough system might store heat for months/years.