Lay your pane of glass on a flameproof surface. Place the clamps on the four corners of the glass, and connect the clamps with rods.
Heat the glass with a low flame from your blowtorch, moving the flame over the surface of the glass slowly. Heat the glass evenly to avoid cracking or warping.
Once the glass is heated enough to be flexible, slowly lift the clamps until the glass has a slight convex surface. Leave the glass to cool and harden for 24 hours.
Check the hardened glass for any imperfections. Heat these with the torch and smooth them out if possible. Allow glass to cool and harden.
Turn the glass over and paint the concave side black. Use a paint that will stick to glass. Apply several coats. Turn the glass around after the paint has dried and look in it. Find any areas where the black paint did not stick and paint over them until you have a smooth, continuous black mirror.
Place the foam in your box and place your mirror on top. The mirror should sit just below the lip of the box, touching all sides of the box. Cover the foam and the visible sides of the box with the black cloth. Place the mirror inside, and superglue the mirror all around the edge of the box. Let the glue set for 24 hours.