Cut the masking tape along center of the top and bottom of the cardboard, using a sharp pocket knife. This opens the box up into a rectangular tube shape.
Slice along one edge of the box with the knife and fold the cardboard out so it’s flat.
Number each large panel 1 through 4. Use a marker pen or pencil.
Cut a small square hole into panel No. 1. This needs to be only large enough to fit a microphone cable through.
Cut all of the foam squares from the top-right corner to the bottom-left corner to create a wedge shape. The wedge shape is important as peaked surfaces typically absorb sound better than flat surfaces. The foam material adds an extra degree of sound-absorption.
Spray all of the cardboard, inside and out, with fire-retardant spray. Leave it to dry for two hours.
Spray each of the foam wedges with fire-retardant spray. Leave the foam to dry for two hours. It’s important that you treat the foam after you’ve cut it. If you spray the squares, then slice them into wedges, you expose the untreated insides, creating a potential fire hazard.
Glue the foam wedges to the inside of the collapsed box. Leave a margin of about one inch on each panel so you can comfortably fold the cardboard back into a box shape. Leave about one square inch of space near each corner on panel No. 3, so you can fit the drawing pins into the cardboard. Leave the microphone cable hole uncovered.
Fold the cardboard panels back into a box shape and tape the edges back together. Since the box is bearing more weight than before, tape the inside and the outside of the large panels to reinforce it. Do this by reaching into the box through the open top. Then fold the top of the box down and tape it shut. Leave the bottom open.
Put a blob of adhesive-putty, such as Blu-Tack, on each corner of the outside of panel No. 3.
Raise the box so it is about head-height. Reach in and push the inside of panel No. 3 against the wall so the adhesive bonds it in place.