A solid oxide fuel or solid oxide electrolysis cell Stack assembly (203) has an improved, simple, cost reducing and robust compression System, housing and Single sided System interface with a flexible-interface-fixture (204) which is rigid enough to fix the at least one cell Stack in the housing when not in Operation, but flexible enough to allow for transfer of the compression force from the flexible compression mat (211) in the top closed end of the housing (201), through the at least one cell Stack and further towards the interface counterpart of the System when in Operation.