How do I cover and close my fireplace opening to conserve heat?

I don't use my open (rectangular) fireplace. I'm sure there is a lot of heat being wasted even though the flue is closed, as they leak air and provide no insulation.
I'd just like to safely fit in some sort of cover to close it up... perhaps rigid foamboard (plus cement board?) with a decent R rating. I'd also like to make it attractive or at least not ugly.
Any recommendations? Anyone done this successfully?
I couldn't find any marketed product on-line.

My daughter has an old brown stone with a fireplace in each room. She is waiting on using them until they can afford relining.
Her fireplaces are all covered as follows:
Measure that pink panther thick as you can get to the size of the opening. Remove the paper backing and glue to sheet rock of the appropriate size. Attach to opening with building adhesive. When fastened to opening, paint the sheet rock the same color as woodwork and put a bowl of flowers or whatever in front.
Her fireplaces look nice and I'm sure no heat is escaping from the rooms.

glue a piece of foam board to a piece of plywood .make foam board the size of your opening and ply about 4 in wider and 2 in taller than the opening. paint something nice on the ply and stick it in the opening. make the foam board snug enough to fit tight

Cut a piece of foam board the exact size of the opening,say 2 inches thick.Tape the edges to the fireplace opening.Then place a nice dried flower arrangement in front.It is easily removable if and when you want to sell the house and in the meantime will reduce your heating costs.

Have you considered closing the top of the chimney instead? You could keep your fireplace for looks and still have it sealed, preventing heating / cooling loss.

If you look up the inside of the chimney you will see a iron gate thing. There is usually a iron ring that you can move the lovers into the closed position.

I have seen people close off the opening with a picture of a fire painted on a board fitted into the opening....but I would bet any nice looking picture would do.

The foam board cut to size and wedged in up in the flue would help block any airflow...easy to remove if needed.