Dubia heating?

[Upirium]

i have a big bin of dubias, who are breeding and functioning but I really want them to grow/breed faster. I have a bunch of TINY babies...
I hear people put undertank heaters on them or whatever but my dubias are in a plastic bin.
They seem to like it but I'm worried if I try to put a heater on a plastic bin it's going to melt so...
what can I do here?