Summarize:

If you don't have a skill or circular saw, a jig, or a workspace that will make cutting your wood and sheet metal easy, an easy way of getting your materials cut is to have them processed wherever you buy them. Most hardware supply stores will cut your materials for free if you supply them with the dimensions. Double-check your measurements, clamp your wood down to a workbench, and measure two points at the top and bottom of the panel to ensure a straight line. Place a template or jig across the line to ensure a straight cut. Clamp the jig down to the workbench and move your circular saw across the wood slowly. If you're cutting from two smaller pieces of plywood or MDF and those pieces are identical, stack them on top of each other so that you only have to make one cut to produce both side panels. Again, add 2 to 3 inches (5 to 7 cm) of extra room for clearance. Clamp your template or jig down and measure a top and a bottom point to ensure a straight line. Start the saw and move it over the panel slowly to produce an even cut of wood. Using the same technique, plan on cutting the top ½ inch (1cm) wider than the sides and 1 (2.5cm) inch longer than the width of the front. This will give the top an elegant differentiation. Depending on how large your radiator is, draw straight lines that are between 3 to 5 inches (7.5 to 12.5 cm) away from the sides and top, and slightly more (4 to 6 inches) from the bottom. This will make the screen grating the centerpiece of the front panel. If you wish to have grated openings on the side panels as well as the front panel, follow the exact same procedure. Because the rectangle you want to cut out of the front panel is in the middle of the panel, you'll need to use this trick to preserve the outer framing. Place down your template to guide your circular saw along the straight line. Position the saw onto the template, with the saw's blade elevated in the air. Pull the guide up from the circular saw, turn the saw on, and slowly plunge the saw into the panel, making sure to leave a little bit of space on the corners. Slowly pull the circular saw along the line until you're about 1 (2.5cm) inch from the perpendicular line. Do the exact same for the side panels if you choose to fit them with grating. Use a handsaw to extend each cut until it hits the corner. This will remove the center piece from the front panel. Cut two 45° inside miters in each piece of molding, so that the four pieces will form a mitered rectangle (reminiscent of a picture frame) when fitted into the front panel.
Consider cutting the panels of your cover at the hardware supply store. Start cutting the two side panels. Cut the front panel. Cut the top. Decide how large you want your grate opening to be on the front panel. Cut the rectangle from the middle of the front panel using a plunge cut. Finish off the corners using a simple handsaw. Measure the rectangle in the front panel and cut your cove molding to fit in all four sides.