A closure device for a containment wall shaft opening is for a milling machine cutter drum assembly including a drum and a drive shaft for rotating the drum, the shaft extending through the wall opening to connect the drum with a mainframe and the shaft and or wall be relatively displaceable. A barrier member is movably coupled with the mainframe so as to be pivotable about an axis fixed with respect to the drum shaft and is disposeable against the containment wall prevent material flow through the wall opening. A coupler is configured to movably couple the barrier member with the containment wall such that relative vertical displacement between the drum shaft and the containment wall angularly displaces the barrier member about the axis as the position of the shaft within the opening is varied so that the barrier member extends across and obstructs the wall opening.