Rock wool insulation is light weight and easy to handle. It can be cut with scissors or a utility knife, creating sheets that are just under one inch thick. Once the rock wool has been installed inside your fireplace it will remain in place for years; also allowing you to make changes if needed because of weathering or deterioration without much effort required.
For a fireplace with an existing stone facade, measure the dimensions of your opening. Cut pieces to size by using scissors or a utility knife and place them in the inside front corner seams. Use any excess insulation as fillers between bricks that are out of line or loose.
Alternatively rock wool can be used for fireplaces without facades where you need to build brick molds: first install thin metal strips on both sides and corners (e.g., sheet-metal water barrier) followed by applied mortar made from sand, cement powder and lime; next go ahead installing rocks with no gaps in between nor left open spaces at their joints; finally cover everything with another layer of mortar but this time it should be crumbly so use it at a thickness of around one inch.