I agree you most likely need to insulate better. That is quite a large bin for so few dubias. You're heating up a lot of dead space. The more dubias there are, the more body heat they produce too.
What do you have your thermostat set to?
70 is still quite cool for a room. The closet I keep my dubias in is usually 75ish which helps a lot. Not to mention I have a lot more biomass with my colony (large number of dubia) which helps too.
I'd tape a towel around the outside of the bin, to help insulate better and lose less heat. If that doesnt work, you might want to get a smaller bin. Luckily they are cheap.
But it just comes down to insulation and heat loss.
-Brandon