Just got back from a pretty exhausting set of medical tests up in Billings, so I'm catching up here.
@SunshineAnShade Welcome to the wacky world of Mama Heating Pad brooding. I think your heating pad is probably just fine. I'm just not sure about using a cardboard box for the cave, but there's absolutely no reason not to try it. This thread has thrived because folks have taken the initial idea and modified it, then shared what they learned.
I have a couple of concerns. That totally enclosed cardboard space with a cut out door is going to get hot inside - probably hotter than a wire frame with a towel over the pad. There is some air exchange with the fabric of the towel - the solid construction of a box blocks that. Using a thick blanket for insulation will only hold even more heat in. And if the blanket holds the heat in, but is thick enough to insulate the top of the box from the heat, will the blanket serve well as a cozy spot for them to sit on when they don't need a lot of heat but want just a little on their tummies and feet? Mine spent most of their time laying on top of the towel, surveying their surroundings and soaking in the warmth. Too thick a surface up there and you will lose some of that. As I've said before, I'm a minimalist when it comes to this system......frame, heating pad, towel or (if they are being brooded outside) straw. Mine also loved that little awning of towel hanging down in the front and the fact that the heating pad was longer than the frame so it extended down the sides of the cave. The towel I used was a worn out bathtowel that was only folded in half, so there wasn't layer after layer of fabric. Using a hard sided cardboard box, they lose that "softness" that accompanies the warmth, and the idea here is to simulate Mama Broody Hen.
You have the basics down - higher in front than back, closed on the sides rather than open all the way around like the commercial ones. And the advice to set it up and test it before the chicks get here is a good one. We'll all be very interested in how you proceed with the box idea. Just remember that you want the pad as close to the chicks' backs as possible for the first week or so. Then watch their behavior. They'll let you know if they are too warm or too cold.
Pictures...don't forget pictures!!!!