Cut the particleboard. Using the saw, cut the particleboard horizontally at 19½ inches. This will be the bottom and top to your new rack. Although they're referred to as 19" racks, some pro audio equipment has clinchers that extend slightly beyond 19", so it's better to be safe than sorry.
Saw the remainder your particleboard in half (you should have approximately 1.2m x 1.4m of wood left). You should be left with four pieces: two that are 1.2m x 19.5", and two that are approximately 1.2m x .7m. These four pieces will be used to make a box.
Nail the boards together, making a box. This should be done by nailing the sides of the 1.2m x 19.5" boards into the top of the larger boards. When standing, the box should have both an open back and front.
Screw your audio equipment directly into the front of the box. The screws go through the front tabs of the equipment and go directly into the sides of the particleboard. Your rack should be approximately 1.2m tall, and it will hold 30 rack units.