A spacer for a membrane stack has an upper surface and a lower surface. The upper surface has a raised perimeter surrounding a membrane supporting section. The spacer has one or more protrusions and recesses configured such that the one or more protrusions of a first spacer fit into one or more recesses of a second spacer with the same protrusions and recesses stacked against the first spacer. Optionally, there may be an interference or snap fit. In a stack, membranes are placed on the membrane supporting sections located between spacers. In one embodiment, the bottom of an upper spacer rests on the raised perimeter of a lower spacer. A spacer may have a hole extending from an edge of the spacer to the interior of a flow field within the spacer. The hole allows access to the flow field for diagnostic testing and/or sampling.