On the Comfy Dory, I used a piece of pine because the grain matched somewhat the laminate material on the cabinets and then stained and varnished it. All I wanted to put on that shelf behind the facing was the radio/CD and to leave the rest of the space for the coiled up GPS cable and our hats.(Both the coiled cable and the hats would launch out of the shelf after running at cruising speed (20 mph) for a short while or moving through rough inlet waters) I hinged the face piece vertically about 1/3 of the way in from the port side and fastened the starboard side with 3 screws into the edge of the shelf.
Make a template before you cut the wood. Measure the width and then take about 5-7 perpendicular, evenly spaced measurements to the bottom edge of the shelf from the cabin roof. Make a paper or cardboard template and then test for fit. You can continue to adjust the template until it suits your eye and then cut your board.
Good Luck...Tad