I just installed a 16X32" pre-made glass block panel into an existing frame in my basement. I had to build out the frame with 1x4s and the glass block panel fit with about 1/4 inch all around. I placed a 3/4" interior stop. Before sliding the window in, I applied a heavy bead of silicon along all sides of the back-stop, and along the angles made by the back stop and the frame, as well as the sill. Once in, I filled most of the 1/4 inch space surrounding the window with silicon, and then I sealed the joints with a silicon bead. Two days later the window is very secure. I was going to place an outside stop, but the window looks so clean I would rather not. My question is Should I? Or will the silicon provide enough stability.