Measuring Planks
Measure your planks so that the outside edge reaches the edge of the panel. You want the cradle to be flush with those edges for maximum support. If you want to recess the cradle a little so it is less visible when you hang the painting that is fine, but the more recessed the more likely the panel can bend eventually. Some artists who work on thin panels like the look of the painting being suspended off the wall that a recessed cradle gives.
Cutting the Planks
Cut the planks at 45 degree angles and join them in a rectangle with wood joiners. Attach the cradle to the back of the panel with wood glue or screws. If you use screws, insert them from the panel side and countersink them, then add some wood dough to cover the holes. This method works better if you are stretching canvas over the panel, as the wood dough filler can fall out in time, taking a piece of your painting with it. If the panel is over 30 inches in any direction, place crossbeams in the cradle. Cut them to the interior dimensions of the cradle, and space them evenly, either in the center of the cradle, or for larger panels from 18 to 24 inches apart. Use a lap joint where the crossbeams meet, and attach them to the rest of the cradle with wood joiners.
Notes on Cradles – To achieve a smooth, consistent edge around the cradle, make the board larger than the cradle and use a router to clean up the edges.