Just wanted to share my experience. I'm not sure if I just wasn't doing it right, or if my heating pad is too fluffy, or if my basement was too cold in general, but I was having issues where the chicks were climbing up in between the wire to get closer to the pad, and it didn't seem like the heat was getting trapped under my cave.. I would find them on top of each other up there. So instead of the "chick cave", I made a "chick house"! I used a 6x12 cardboard box and attached the heating pad to the sides and roof using HVAC repair tape, which is heat-proof. I did not fold the heating pad at all, just draped it. They really love it and settled down immediately and seem to enjoy laying against the fluff. There's room to cut the doors out so they can get in there while they grow, and I can also just move it to a bigger box when needed.
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Although the idea is indeed to have the pad right at their backs, as far as this cookie is concerned if it works, go with it!

You can see my latest batch of Silkies in this shot, living outside in Northern Wyoming and loving it. A lot of us have changed over to putting the heating pad on the underside of the frame. Less chance of chicks getting trapped between the frame and the heating pad. There are good, clear directions for doing this right here:
http://www.backyardchickens.com/t/956958/mama-heating-pad-in-the-brooder-picture-heavy-update/450
As for temperatures under a set up like the one pictured below, the average temp under there on the floor was only 82.5 degrees in a 69 degree room!
@azygous asked about that very early in the thread, so I decided to measure it and give her a definitive answer. Going by the book, my chicks should have been dead, but there it was on Day 6 under there and they were thriving. Since then nothing has changed for me and mine besides where they are raised, and for me that's now outdoors in the run from the start - and our springtime temps out there are still in the teens and twenties many times, with snow! Since the pad is only warming the chicks by direct contact, usually the ambient temperatures don't matter that much - within reason, of course.
After even a few days, but definitely after they are about a week old, they always spend as much time on top of the pad, if not more. They like it. It's soft, it's cozy, it's warm, and they can survey their domain or play king of the mountain.
I'm not trying to talk you out of doing something that's working for you. I'm just kinda reassuring you that at one point or another we all entertained some doubts about how this worked and had a bit of a "relearning" curve. Good luck with your chicks! Oh, and your Broody Brigade membership dues are paid! <grin>