Im judging by the way you worded your question that you are looking for an inexpensive option to make your fireplace more efficient. The best way to do this is by having it fit for a fireplace insert, but that is far from an inexpensive way to tackle this issue. Also, most of the quality insert manufacturers are running 4 to 6 months out for delivery of their products anyway, so even if you did have the cash on hand for that you wont get it until spring anyway.

I can speak from experience that the other blowers and do-dads like firebacks, heatelator fans, and heating tubes, etc… are gimicks that essentially give you nominal or no heating advantage. Over the years I have tried a host of them and I cant say I have found one I can recommend. The simple fact is fireplaces are inefficient at heating a home, period.

You can try the box fan idea, but you may cause smoke and sparks to draw into the room as well. Be sure to keep the spark screen up if you do try it, and dont leave it unattended at all.

If you are truly looking to generate usable heat for your home. The best thing you can do with a open fireplace in the winter is plug the thing with a chimney balloon so the damper isnt letting out inside heat and save your nickels for an insert or upgrade your furnace to a high efficiency power vent unit. Any wood you burn in the open fireplace will be wasted and worse yet you will be wasting your furnace heat as well.