My SmartMatrix 32x32 LED display is mounted inside a square picture frame and held in place with two fasteners. There is a gap between the outside border of the display and the inside border of the picture frame, which lets light through from the rear. It is especially noticeable when it is sitting on a window sill with daylight behind it.
Using OnShape I designed a solid plastic border that fits around the display and holds it in the frame. The border was a little too big for the printing bed of my Prusa i3 MK2 to print out in one piece, so I split the design into two parts which I printed out separately. I also have an IR receiver at the top of the frame for an IR remote control so I printed the top half of the border with a notch to accommodate the IR receiver's leads.