Starting Dubia Colony, Need Help!

Hello!
So I just ordered 3 females and 1 male dubia hoping to slowly but surely start a colony. I have a plastic bin that I will be keeping them in with egg crates and everything. But I was wondering how I should keep their plastic bin heated? The reptile tank heater mats all say to only use on glass, not plastic. I guess that makes sense because applying direct heat to plastic may release harmful toxins/chemicals of some sort and I don't want to kill my dubias. I was wondering if a normal medical heating pad from the pharmacy section in Walmart would work? Or does that not emit enough heat? Would a ceramic light work on top of the lid or would that cause the same problem? I live in a room that is maintained at about 70-75 degrees.
Also, I don't know what to do for hydration. I see most people using water crystals, but I'm very wary about them. My last bearded dragon somehow swallowed one and died when I used to feed him crickets, even though the cricket tank and the beardie were kept separate. I'm assuming either one was attached to the cricket somehow, or Petco had something to do with it since I only had him for 3 days...my current dragon has been doing really well though without any water crystals involved. How else can I keep my dubias hydrated?

Thanks for the help!

That's a very small number to start with. It may work if they stay healthy but it could take quite a long time to get a colony established. You are probably better off with several hundred mixed sizes in addition to your adults in order to stagger generations a bit. Either that or add more adults (maybe 5-10x what you have)

They can do OK at 70-75 without heat. They thrive when it's a little warmer but based on my experience they will do fine at regular room temp too. They are pretty hardy. In the winter I use a ceramic heat emitter hanging from an adjustable lamp stand that sits over the roach tank to bring the temps up a little (mine are in the basement which goes into the 60s over the winter).

For hydration and food I just offer them fresh veges and occasional fruit. They eat up the leftover salads from my dragon. They LOVE squash and slices of orange. I just kind of feed them the same diet I give to my dragon. As long as you keep the veges fresh and remove leftovers after a couple of days (if there are any) they should do pretty well.