A planar frame of cardboard or other suitable sheet material with an interlocking corner construction. The corner connection provided in this invention securely locks the corner in place at low manufacturing cost. It can be integrated easily into the filter frame assembly process without addition additional steps. While the locking mechanism is secure to hold the frame in place for transport, during manufacturing, and for final filter usage, it is removable to allow for the frame to be unfolded and reassembled if necessary during manufacturing, e.g. to insert filter media. Preferably, after inserting filer media, the tabs hold the filter and frame in an assembled configuration until the frame can be sealed with adhesive. In a preferred embodiment, the frame comprises three simple, rectangular tabs. The first tab is attached to the first frame leg and simply slides between the outer and inner frame parts of the second frame leg. The second tab, which is small in width (approx. 1/3 to 1/2 of the width of the frame leg width in plane of the filter medium) and extremely small in depth (approx. less than 1 mm), is attached to the first frame leg and inserted into a small slit in the adjacent second frame leg during assembly. Due to its small size, this tab locks the corner in place but barely protrudes to the outside of the frame. The corner can be disassembled without destroying the tab by pulling the second leg to the outside. The third tab holding the corner in place is located at the second frame leg at the outside border of the scored blank. This tab is inserted into a fitted opening located in the inside fold of the first frame leg. This tab holds the frame corner in place but can be popped out of its fitted opening by pulling the first frame leg to the outside with a specific force. Alternative embodiments may contain the first and second tab only, or the first and third tab only.