The panel is generally made from tulipwood planks which are first glued toether to the size of the icon.
These glued planks are then shaped to square and grooves cut into the back of the panel so as to fit hardwood (usually oak) trusses. These trusses are designed to prevent the panel from warping.
The front of the board is shaped with a shallow indent for the painting.
These same principles apply to any shape of panel...