A cassette for containing a gel slab used in an electrophoresis separation comprises a pair of spaced glass or like material plates having spacer strips interposed between marginal side edge areas of the plates to define a space for containing a gel slab between the plates. The spacer strips are made of a flexible double rolled smooth finished polymeric, preferably polyvinyl chloride material which when used for such purpose function to interface under pressing pressure imposed thereon to exclude atmospheric air at the interface so that once interfaced with the plates, atmospheric air pressure acting on the plates maintains a pressed sealed interface condition effective to prevent liquid flow from a liquid-containing gel slab composition cast in the cassette sideways through the cassette during gelation of the composition. An assembly tool and method for effecting cassette assembly is provided.