Lay the boards on the floor in pairs. Line them up until the corners are flush.
Place a hinge over two of the boards, approximately 6 inches from the top. Poke a sharp pencil through the mounting-holes to mark their position. Place a second hinge over the boards, approximately 6 inches from the bottom, to mark the mounting-hole positions. Repeat this process with the other two boards. In order for the baffle boards to be free-standing, they must be hinged together in pairs.
Drill two 1/8-inch mounting-holes in each board, using the pencil markings as a positioning guide.
Place a small block of wood under the top edge of the board to elevate it slightly from the ground.
Screw in the hinges. Repeat the elevation and screwing process so the boards are hinged together in pairs, at the bottom and top.
Remove the block and lift up the boards so they stand vertically. Carefully lower the boards so they lie hinge-side-down.
Cut each foam square into a wedge shape using a kitchen knife.
Douse the foam wedges with flame retardant spray. It’s essential to spray the wedges after you’ve cut them. Spraying the squares, then cutting them into wedges exposes the inside of the foam, which isn’t treated with flame retardant. Leave the wedges to dry for 2 hours.
Glue the foam wedges onto the non-hinge side of the boards so that the narrow end of each wedge is pointing in the same direction. The wedges create a peaked surface, which is ideal for absorbing sound reflections and “deadening” the sound of the amp. The foam material is particularly absorbent.
Stand each hinged pair of boards up and position them around the amp so they face each other, creating an open-top box shape.
Drape an old blanket over the top of the boards and tape it to the outside of the boards.