My gas forge has a fire brick in the bottom surrounded by kaowool with a refractory mortar coating. I am using a 1/4" thick sheet metal with slightly curved up edges, except the edge facing me which hangs out of the forge. That way the only side the flux can run off is outta the forge. Then just pull out when done with a pair of tongs. Needs to be replaced once in a while, but very cheap to replace. Saves the fire brick from being eaten away.