Could you maybe use some sort of cave? Or a wooly hen? I brood mine outside using the heating pad, and a cave made of rigid foam insulation. It retains their heat quite nicely, especially when there were 13 underneath.
I've also seen posts where they use a wooly hen. Think of a shoebox with straps of wool or Fleece hanging down for the chick's to snuggle in.
I've had two batches of chick's. My first one came in the mail and two didn't make it. I think one was squished in the box during transit, and the other died within 24 hours. 2nd batch came from a feedstore and one was weak from the start, never ate or drank and didn't last long. It doesn't seem that uncommon to have a weakling who doesn't make it.
Best wishes for you and your chick's!