PerkinElmer supplies rectangular spacers made of either lead or teflon. These spacers come in a variety of thicknesses, allowing you to customize the Universal Demountable Cell's path length to the experiment's requirements. Also available is a packet of Assorted Rectangular Spacers (L1270421) which includes two of each type of spacer.
Please note: When rectangular windows and spacers are used, the Universal Demountable Cell becomes a Semi-Demountable Cell.