As an electrical engineer I would recommend against making the light boxes. Putting a 300w bulb into a fixture not rated for it is risking overheating, shorting, shock and/or fire. The soft boxes have no means of ventilation and the heat could cause fire and/or toxic fumes from the boxes used. Even if you punch ventilation holes, the radiant heat from a 300w bulb or group of 4 bulbs can easily be too much for the cardboard/plastic materials nearby.