I personaly heat the area they're in. Instead of attempting to heat individual containments. Aquareums are different. You can do those rather seperate. But if you have a bunch of critter keepers, vails, cups and jars. Just heat an area.
If you have more than one plastic container
Get a suitable size fish tank, put as many in it
as will fit, then use the heat mat to warm the whole thing! Presto, instant warm spot for all.
Last year when our power went out, I did this
and plugged it into the generator ( along with
the freezer) NO living thing was lost, No
frozen stuff lost either.=D