A rigid self-supporting filter unit is produced by placing two portions of fiber fabric in face-to-face contact and stitching them along parallel regions, using two rows of stitches (16), to form pockets into which formers (20) are inserted to shape the pockets into a generally diamond shape. This assembly is then placed between two presser members (22, 24) which are corrugated, the "valleys" of the corrugations receiving the diamond-shaped formed portions. The presser members are heated and the portions are thus treated by heat and pressure to cause the fabric to be rendered rigid. The formers can then be removed. Suitable fibers for this purpose include a rigidising fiber known as P84, polyphenylene sulphide fibers, polyester fibers either alone or mixed with polypropylene fibers, and bicomponent fibers.