For the barrel oven, the over used for the main part of the barrel to form the arc shape is a tricky affair. For the mainstream barrel oven construction procedure, the formwork used is costly hence; it is recommended you use locally sourced items, like the barrel cover. The barrel cover can be used wholly or it can be cut to for a semicircle. In some barrel cover oven, the top part is left open, which posses the risk of burn-accidents. It is recommended that the barrel cover be covered with brick, insulating materials, etc. moreover, these materials help to prevent heat loss.
Further information: Make your own barrel brick pizza oven and outdoor setting