A sealing case for a filtration cassette that reliably prevents leaks after assembly and installation into a filtration system. The sealing case fits on the filtration cassette and provides a fluid-tight seal between the sealing case and around at least one port in the filtration cassette when installed in a filtration system. The sealing case may comprise two halves that mate together, and each half may be identical. The internal surface of the sealing case is flat or substantially flat.