which extends perpendicular or at an angle to a front edge of the gable plate and which has a shoulder 156 the complementary retainment member 146 comprises a recess 158 which is arranged at the rim 118 of the front plate 110 the shoulder 156 has been received in the recess 158 thereby the gable plate 112 is in fixed engagement with the front plate 110 however in order to further enhance the stability of the engagement the retainment member 144 further comprises three protrusions 160 which protrude from the front edge portion 128 perpendicular to the primary extension of the gable plate 112 the protrusions 160 are distributed along the length of the front edge portion 128 the complementary retainment member 146 further comprises three notches 162 which are arranged at the rim 118 of the front plate 110 one notch at each respective protrusion the protrusions 160 have been received in the notches 162 each protrusion 160 is wedge shaped and each notch 162 is v shaped the sizes of the protrusions 160 and the notches 162 are such that they are connected to each other in a press fit when the gable plate 112 is inserted into engagement with the front plate 110 the snap lock elements 148 and the press fit connection between the protrusions 160 and notches 162 ensure a reliable coupling between the gable plates 112 and the front plate 110 even without the use of any adhesives such that the filter frame can be handled e g lifted and moved.