Remove the cover plates, or pull the insert out just enough to give yourself enough space for stuffing insulation into the gaps. Since most inserts are very heavy, use a helper for moving the insert.
Stuff fireproof glass wool insulation into the fireplace. This is not normal fiberglass insulation, which is only fire resistant up to 212 degrees Fahrenheit, but rather a product that can withstand the intense heat of a fireplace insert up to 1220 degrees F. Some manufacturers create a self-adhesive side on a roll of wrapped fireproof insulation. Place the adhesive side around the insert.
Slide the insert back in place if necessary, or reinstall the cover plates. Stuff loose insulation into any remaining gaps around the fireplace mouth.
Ask your chimney professional to reinsulate each time he pulls out the insert to check and clean the chimney, usually once a year.