Hi Jerry,
I'm glad it's working -- I did this for years, but more elaborately.
Cardboard is great at containing heat.
There are lots of ways to raise chicks without electricity. My favourite is a wood-sided 'cold brooder' or '
hay box brooder', commonly used in the 1930s.
If you ever find yourself battling lower temperatures, I can vouch for a haybox brooder, properly built (4
wood sides with drilled vent holes near the top along each wall and a doorway in the middle of one side, a circle of mesh inside attached to the door-hole, and hay stuffed between circle and box walls; drape a hessian sack on top to make it cosy; put it somewhere safe from rats or other predators and under cover). I've used a hay box brooder down to just below 40F outside (overnight). You have to make sure there are
enough chicks to fill the mesh circle and you have to let them out and pop them back in (closing the door or stuffing something into the entrance) four times a day for the first week, then they learn how to do it themselves. As they grow, you raise the box on blocks, and/or increase the size of the mesh circle. It's hard work for a week, then it's easier than anything and best of all, no worries if the power goes out.
Anyhow, what you're doing proves there are lots of ways... And as long as there's a balance between ventilation, insulation, and chick numbers (you can't cold brood small numbers of chicks so well because they're not producing enough heat), and as long as you know your weather, you can do just fine with no extra heat.
cheerio
Chancy