preferably substantially alike and the following description relative to the attachment at the overlapped portions of legs 95 and 103 should suffice for all hole 121 extends through web 57 and is preferably spaced approximately one inch from the distal end of leg 95 screw 119 may be inserted through hole 121 since screw 119 is self tapping when it is turned it will rupture rib 85 of female section 51 as seen in fig9 so that legs 95 and 103 are locked together then if it is ever desired to disassemble filter 11 the screws 119 may be removed from the foregoing it will be apparent with the coaction of the projections 63 in sections 47 49 and the slots 83 in sections 51 53 a very sturdy and rigid filter 11 is provided which is easy to assemble thus the coaction of the projections 63 and slots 83 permits lengthwise telescoping movement of the legs of the male sections and the legs of the female sections relative to one another and yet prevent inward separation of the legs of the female sections from the legs of the male sections it will be understood that when the term 34 preventing inward separation 34 is used herein it will be deemed to mean preventing the separation of the legs of the female sections from the legs of the male sections in a direction toward the center of the filter i e in a direction which extends perpendicular to the longitudinal axes of the legs of the sections also there is provided a filter 11 which is very efficient in operation due to the large interstices 35 and which can be easily trimmed to size due to the narrow legs 33 of the squares 31 as well as.