Determine which direction you want the glass to slide out. This section of the box will need only the decorative molding in place, not the narrower channel spacers.
On the three sides which will support the glass glue a strip of 3/16 inch wide basswood which is slightly thicker than glass you intend to use. These strips of wood will be the channel spacers. The 3/16 inch width will be glued even with the outer edges of the plywood roombox, leaving a 1/16 inch ledge on the inner edge of the box to prevent the glass from falling back into the display. Glue these pieces so they line up with the outside edge of the box on all four sides. Only the end of the piece from the adjoining side will line up with the outer edge of the box on the opening side. Check your pre cut piece of glass fits between these three channel spacers, or measure the opening with the channel spacers in place to determine the measurements of the glass you will need.
When you know your glass fits and will not fall back into the box, take a piece of wider strip wood or a piece of half inch picture moulding, and cut lengths that will fit together with mitered corners to cover the top or front of the box, if set even with the box's outside edge. These pieces of molding will cover all four edges of the box top or front and will hold the glass in place on the front like a frame. Test fit all four pieces to make sure they are square and line up properly. Glue the molding together in the corners to form a frame. When the frame is dry, glue it to the channel spacers, checking that the corners of the molding on the edge where the glass slides out are securely glued to the channel spacer on the top and bottom. If you do not need to add a molding to the opening side, you can cut the top and bottom molding sections square with the open edge, just like you cut the channel spacers.
