A seal includes a body and two lips. The body includes two walls defining a space with an opening. The lips have fixed ends respectively connected to the walls adjacent the opening. A movable end of the first lip is within the space. A movable end of the second lip extends across the opening towards the first lip fixed end. The movable end of the first lip includes a first end portion extending from the first lip fixed end along the first wall to a middle portion extending across the space to a second end portion extending along the second wall towards the second lip fixed end. The second end portion includes a hook section meeting the second wall and the second lip fixed end. The second lip movable end moves into the space and presses the hook section against the second wall to clamp the hook section.