Abstract: In one embodiment, the invention is a register assembly for inserting into floors, ceilings and walls, comprising a first open top box having a bottom surface, which defines a centrally disposed opening through which air flows from a first environment to a second environment. A support is disposed within the first open top box. A second open top box nests within the first open top box, and is supported by the support. A restraint is associated with the first and second box to restrain the second box within the first box. The first and second open top boxes are sized and dimensioned to be coplanar with each other when nested. A conduit is also defined between the first open top box bottom surface opening and the second open top box to flow air from the first environment to the second environment.