I had the same problem!
Because I use large cardboard boxes for my brooders I didn't trust the strength of the cardboard to hold a pole across the top with the lamp clamped to that.
So what I did with my first batch of chicks was go get an old floor lamp out our attic and put that in the box with the lamp attached to the top part that swings out. This was back in April and I couldn't get it warm enough in the box with the lamp on the outside, so I covered the base of the lamp with plastic and stuck it right in the box in a corner. The chicks all thought it was a toy too, they would stand on it and hide behind it. Even though I was afraid they'd eat the plastic they never did.
For my newest chicks, I am using a tall plant stand. It stands about 5 feet tall, and the arms for hanging plants are perfect for hanging the lamp from. I used wire to attach the clip on the lamp to the arm of the stand, and also have the cord wrapped around it in such a way that the lamp can't fall into the box. Because it is summer now, I have the plant stand outside the box. I just have it in one corner, so they have plenty of room to move away from the heat if they need to.
You could take 2 chairs, one on either side of the box and tie a pole or a broom or something to the tops of the chairs then hang the lamp from that.
In another post I wrote that with the heat here, even though my babies are only 7 days old, I've been having to turn the lamp off during the day, it is just too warm for them.