I saw on another thread that you have your heating pad set on 6. This setup might be fine for a day or so, but you will be amazed at how fast they'll grow - sometimes it seems like you can stand next to the brooder and hear them growing!! So I think what I'd do first is turn the heating pad down. It's pretty close to their backs, and your set up is indoors and closed within a solid cardboard box, so they don't need it that high. Chicks (and adult chickens) don't do well with too much heat, despite the dire warnings to keep them super warm at all times. They get lethargic, just like we do if we are sitting on the patio in 100 degree heat with no way to cool down. So turn your MHP down to about 4 and see how they do....I'll bet they'll come out a little more and explore a little more if they aren't overheated. If they start bunching up and acting cold, you can gently pop them back under and see if they relax.
I know you said this was temporary...what will your next brooder be like? I think the next thing I do is try to make a frame that's adjustable. It doesn't have to be fancy - just a piece of hardware cloth with the sharp points taped over and some dowels stuck into it across the top to help it keep its shape would be just fine.