It should touch their backs- maybe put it a little higher? I made my frame from a piece of heavy wire fencing. I really like this because it is higher in the middle and gives a variety of heights as it slopes to the floor on the sides. I also squashed the back lower, making sure to leave the back open a little so the chicks can get out the back if they want.
The varying heights allows the chicks to choose the right amount of heat for the conditions. Mine are in an unheated building, and during 20 degree F. nights I noticed they crowd against the side parts to get maximum contact with the heating pad.
Another nice thing about the flexible wire frame is that I can make the middle taller as they grow.
I think it is very important to make sure the chicks can't get in between the wire frame and the heating pad. I put my pad under the frame and closed the whole thing up inside a pillow case. You can use wide tape or bungee cords to keep the pad up against the frame. I also made sure the whole opening of the pillowcase was taped shut.
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I put straw underneath and on top to help insulate the cave.