I would do exactly what you said, move them out with one optional heat source, only turn on at night. That's a lot of chicks, hope you have a big space! I've been able to hang a ceramic heat bulb from my coop roof by a hook with an extra clamp (strong, large size metal binder clip,) just to make it double secure, in the past when I had chicks in *20 and I was new to the MHP. Since then I've only used the MHP as I think they don't need 2 different heat sources. Can you make the cave taller? If they can get under it and choose not to then they don't need it or it's too hot. Which ever heat source you choose, I'm betting they wont need/want it for much longer at all. Hope it all works out!!