What size bin for dubia colony? Other newbie ???

I think we are going to take the plunge and start a colony. I have people who would get roaches from us if we are overrun with them. I was looking to see what I had on hand that I could use to set up the colony. I have two solid colored bins that I could use. One is 21 gallons and the other is 37 gallons. Which one would be best for our colony?

I was thinking of ordering 40 adults (30 females and 10 males) to get started and maybe adding in a few mixed sizes with them.

Also, if I am to use one of these bins and wanted to use heat tape what size would I get? I assumed I would put it on the outside but I thought I read on here that some people put their heat mats on the inside of the bins....perhaps I read that wrong.

Any other tips/suggestions are appreciated. We have been using them as feeders so we are familiar with him and know that our little guy likes them.
Thanks,
Jamie

Either size would work well. I use a 20g glass tank with mesh top and it's fine for my small colony. I have it sitting on a bakers rack to allow ventilation and put a heat mat right on the bottom. I haven't used the tape, so I'm not sure what side I'd use. My heat mat is on one side of the tank underneath the cardboard housing. I keep the other side of the tank unheated and put the food on that end.