A fastening device (NUT1, NUT2, NUT3) suitable to be used for forming an interlocking push-in coupling between a first polymer containing product (PROD1, PROD11, PROD12, PROD13, PROD14, PROD15) having a protrusion (APP1, APP2) and a second polymer containing product (PROD2) having a hollow section (GAP1), the fastening device comprising: - a base plate (BAS1, BAS2, BAS3) having a first pair of opposite sides, a second pair of opposite sides, a first surface (SURF1) and a second surface (SURF2); - a first pair of blade elements extending from sides of the first pair of the opposite sides; - a second pair of blade elements ; whereby - the first pair of blade elements is connected to the base plate by a first pair of flexible joints forming a first acute angle α between the normal (N1) of the first surface (SURF1); - the second pair of blade elements is connected to the base plate by a second pair of flexible joints forming a second acute angle β between the normal (N2) of the second surface (SURF2); and - when mounted on the protrusion (APP1, APP2) of the first polymer containing product (PROD1, PROD11, PROD12, PROD13, PROD14, PROD15), the first pair of flexible joints is configured to press the first pair of blade elements against the first polymer containing product (PROD1, PROD11, PROD12, PROD13, PROD14, PROD15) to form a first joint (JOINT1); and - when pushed into the hollow section (GAP1) of the second polymer containing product (PROD2), the second pair of flexible joints is configured to press the second pair of blade elements against the second polymer containing product (PROD2) to form a second self-tightening joint (JOINT2); characterized in that - the second pair of blade elements extends from sides of the second pair of the opposite sides; and - the first joint (JOINT1) is a self-tightening joint. - a base plate (BAS1, BAS2, BAS3) having a first pair of opposite sides, a second pair of opposite sides, a first surface (SURF1) and a second surface (SURF2); - a first pair of blade elements extending from sides of the first pair of the opposite sides; - a second pair of blade elements ; whereby - the first pair of blade elements is connected to the base plate by a first pair of flexible joints forming a first acute angle α between the normal (N1) of the first surface (SURF1); - the second pair of blade elements is connected to the base plate by a second pair of flexible joints forming a second acute angle β between the normal (N2) of the second surface (SURF2); and - when mounted on the protrusion (APP1, APP2) of the first polymer containing product (PROD1, PROD11, PROD12, PROD13, PROD14, PROD15), the first pair of flexible joints is configured to press the first pair of blade elements against the first polymer containing product (PROD1, PROD11, PROD12, PROD13, PROD14, PROD15) to form a first joint (JOINT1); and - when pushed into the hollow section (GAP1) of the second polymer containing product (PROD2), the second pair of flexible joints is configured to press the second pair of blade elements against the second polymer containing product (PROD2) to form a second self-tightening joint (JOINT2); characterized in that - the second pair of blade elements extends from sides of the second pair of the opposite sides; and - the first joint (JOINT1) is a self-tightening joint. <EOS>