I've been in the process of designing my "chick cave" for my anticipated six new babies in May. It involves making a "u" frame out of foam from a cheap camp sleeping mattress from Wal-Mart, with dowels to hold it in its "u" shape as well as acting as supports for the "roof" which is the heating pad with towel over it. I plan on gluing heat reflective metalized poly, (otherwise know as a space blanket) on the inside walls of the foam "u".
This design can be built in any size to accommodate an number of chicks. One heating pad would not need to actually cover the entire area beneath since the heat radiating downward will also be reflected off the walls. I haven't built it yet, but I know it works because I made an enclosure for my meal worm farm with the walls, floor, and ceiling all lined with the space blanket material, and it keeps the wigglies warm for 24 hours by simply heating a couple tubs of water once a day and placing them inside under the worm trays. For two or three dozen chicks, I believe you could build a large cave and only need two carefully placed heating pads to adequately heat the large space beneath with the poly lining the walls reflecting heat back onto the chicks.
The space blanket is dirt cheap and can be found in the camping aisle at Wal-Mart. I was thinking that this stuff is so cheap, mail order hatcheries should line their chick shipping boxes with the stuff and there might be less mortality, especially during the colder months. I plan on using it in the box I'll be bringing the chicks back home in when I get them at the post office when they come in.
I'll post photos when I get it built. Don't expect it right away, though.