Abstract: A lockable assembly for hermetically sealing an opening consists of a threaded wing bolt, a domed capping plate having an annular axial hole and means for locking the assembly, an elastomeric sealing gasket, and a base plate having a threaded axial hole for receiving the threaded bolt. Positioning the gasket between the capping and base plates, the bolt is inserted through the assembly, then base plate first the assembly is inserted into the opening to be sealed. A flange on the gasket and the capping plate support the assembly on the rim of the opening to be sealed. Screwing the bolt into the assembly urges the plates toward one another compressing and deforming the gasket radially outward creating the seal. Accompanying frictional forces between the gasket and inner surface of the opening to be sealed along with the locking means prohibit the removal of the assembly by unauthorized persons.