I have read a lot of articles and videos on rocket stoves and how to get more heat our of your wood stove. I paid $2.50ea X 6 pieces for the fire brick buy the heavy solid style not the light insulating kind. It holds the heat better and takes a long time to cool down. Then just placed it in wood stove to take the chill off I was amazed how well it worked.
Use soft wood and cut it into small pieces. Hardwood has too much coal and ash.
To make it interesting the next step is to surround the insert with river rock to absorb the heat and the one hour burn will warm you for hours. I will run the test and post the results.