A multielevational cover for assembling a fluid retaining chamber on a face of an electrophoresis cassette is presented. The cover comprises a cassette-containing member, at least one chamber-defining member, and sealing means, typically sealing means that are capable of reversible attachment. Also presented are methods for contacting cassette-immobilized gels to desired fluids by assembling a fluid retaining chamber upon the cassette using the covers of the present invention.