Plastic with screen lid, like their parents were kept in.
I know a lot of the die-off is due to the unexpected hatching, but I wonder if the foodplants were insufficient. The wild population was found on some low plants around an apple tree, and they ate rose and blackberry after I took them in. But I wonder if some nutritional deficiency happened with these nymphs. The reason I didn't feed them those un-ID'ed wild plants was that they haven't grown in yet...
In what may be a related incident, I once raised some Microcentrum katydids on maple alone, and compared them to ones which were fed a variety of foodplants. The maple group ate a basically equivalent amount of foliage, but grew much more slowly and experienced "random" deaths which didn't occur in the group which ate several plant species.