Soft material that you intend to make into felt is muktza on Shabbos “machmas khesoren kis” since you are makpid not to use it for anything else. If you designate it before Shabbos to use for insulation (hatmanna), it isn’t muktza anymore. The designation can be mental but it must be designated forever. (MB3,5)
Stones and bricks on top of a stove are not muktza – they have been designated for a permitted use. (MB11)
It is ossur to seal (a pot of food inside) a coal oven. The air gets blocked and chokes the red hot coals which extinguish them. Even though it is a p’sik raysha d’lo niche lay, it is still ossur d’rabbonon. You are allowed to tell a non-Jew directly to seal the oven. We aren’t makpid about amira l’nochri for a p’sik raysha. (ShA7 – MB21 – ShaH16)