I agree with the above. Those 250w bulbs can easily flood that box with too much heat. I use one but my brooder is about 2’x4’, the walls are made of hardware cloth, and the brooder is outside. Just make sure, using a thermometer, that the chicks can get away from the heat if they want. My heat ranged from 95 degrees down to whatever the outside temp was at the time, maybe 40 degrees some nights on the opposite side from the lamp. My chicks never slept directly under the lamp, they always huddled up somewhere away from the heat. A laser thermometer is a very useful tool when using a heat bulb that powerful.
It looks like your brooder is in a closet? If so, consider using the cardboard box as the floor with paper towels as the bedding. That will give them more room to get away from the heat if needed. You can cut a piece of hardware cloth to wedge into the door frame to act as a wall to keep the chicks in.