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.
After reading through quite a lot of this thread, we went with the MHP for our chicks. We started with the heat on 6 when the chicks were indoors and the chicks ranged from 4 or 5 days to about 10 days (according to the woman at the store). The room was about 65-75 degrees during the day and they spent the whole day out running around. They seemed happy, so we decided not to worry about it. As they got bigger, the night-time temps in the MHP grew quite warm - so it wasn't long before we dropped to setting to 4 and then 3. (We have a wireless thermometer in with them)
We moved them outside last weekend once the temperatures were expected to be in the 60s 70s consistently. Yesterday was very windy, which worried me some, but they just camped out in the enclosed upper part of the coop, occasionally ducking in and out of the MHP, and seemed perfectly happy. (Is it weird that we have a webcam on them so we can watch them all day? )