Abstract: An adjustable mounting frame for supporting a storm glass assembly exterior of a prime assembly along the clearance edges of a wall opening in a finished building construction utilizes a pair of interdependent frame and support flange pieces. The support flange contains a reversible first surface for facing flush against the exterior portion of the wall and an orthogonal second surface having longitudinal engagement members for connecting with the frame piece. The frame piece has a first side facing the second surface of the flange formed with a plurality of connection elements for slidably attaching the frame piece to the flange and a second side facing into the opening with mounting groove means for supporting the storm glass assembly.