A filter contains fluent particulate, such as carbon granules. The filter is divided into a plurality of cell chambers by walls, including an upper wall. The upper wall has a surface that faces, and is closest to, the top surface of the particles. The upper wall surface is oriented so that, as the particles settle, any gap formed between the top surface of the particles and a portion of the upper wall surface will not extend from the upstream to the downstream sides of the filter. Instead, contact is maintained between the top surface of the particles and the upper wall surface during fluid flow through the filter, and therefore any gas flowing through the frame must flow through, and through spaces around, particles. An angle between the upper wall surface and the flow direction of gas striking the filter is greater than about three degrees.