Flat plates serving as molds and enclosures for slab gels are held together by a clamping frame in which the side edges of the plates slide into facing channels and are clamped by lever-operated cams that compress the channel walls against the plate surfaces. The joined plates are held on a vertical support rack containing a finger-operated spring-loaded notched clamp that presses down on the plates to seal the opening at the bottom edges of the plates against a gasket. A well-forming comb for insertion between the plates contains flexible outwardly angled fingers to seal against the spacers between the plates.