first and second plates, both substantially rectangular with side edges and end edges, and

means for joining said plates parallel to and facing each other with a gap of preselected width between said plates while closing said gap along said side edges and leaving said gap open at said end edges for electrode buffer access, the improvement in which said means for joining said plates comprise a plurality of edge engaging members distributed along said side edges, each said edge engaging member mounted to one plate to engage an opposing side edge of the other plate in a manually releasable manner, wherein each of said edge engaging members has an inverted shoulder to receive said opposing side edge and is pivotally mounted to pivot said inverted shoulder into and out of engagement with said opposing side edge.