Abstract: A window buck is formed of side walls that are extruded from vinyl. The side walls have interior channels. The side walls may be formed to effect a female-male connection between the ends of two adjoining side walls. Alternatively a locking member is provided that is sized to frictionally engage a selected channel. The edges of the side members are shaped for abutment to each other with a connector in the channels to hold the window buck together. The window buck thereafter may be placed before the cement wall is formed and is sized to receive a window frame. The window buck may be formed into selected geometric shapes. Window bucks of different dimensions may be assembled by selected sides of different dimensions from bins or pallets of presized side members.