Two identical arch shape cross section chambers are joined together so that a dome shape part of one end of the chamber is overlapped by the opposing plain end of the second chamber. Thus, one chamber may pivot in the horizontal plane, for adjustment during installation. A latch at the top of the joint between the chamber pair inhibits vertical motion and separation of the overlapping chamber, while permitting horizontal plane pivoting. A latch comprises a tang which cantilevers outwardly from the top of the dome end, to engage a catch which is a flared portion of lip on the overlapping plain end.