Measure the cutout area in the countertop where the cooktop will rest. Make a note of these measurements and add 1.25 inches to all sides. Then measure and take note of the depth of the countertop.
Draw the original dimensions on a piece of stainless-steel sheet metal. Then draw another rectangle that is 1.25 inches wider on all sides. Draw a third rectangle within the first one you drew. The difference between the two innermost rectangles should be the dept of the countertop on all sides. For example, if the depth of the countertop is 1 inch and the first rectangle is 20 inches by 30 inches, the innermost rectangle will measure 19 inches by 29 inches.
Cut the sheet metal using tin snips, with the lines of the largest rectangle as a guide. Wear leather gloves tailored for handiwork so you do not accidentally cut your hands as you work.
Cut out the innermost rectangular shape. Hammer the end of a flathead screwdriver into the middle of the smaller rectangle to punch a few connected starter holes. This process will make it simpler for you to use the tin snips.
Smooth out the cut edges using an angle grinder. Wear safety goggles so metal filings do not get into your eyes. In addition to smoothing out the edges, you can use the angle grinder to straighten any irregular lines that you cut with the tin snips.
Use the tin snips to cut a straight line from the innermost corners of the trim kit to the corners of the middle rectangle you drew.
Using a sheet metal folding tool or a locking vise grip for sheet metal, fold the sheet metal in the innermost rectangle at a 90-degree angle, with the line you drew for the middle rectangle as a guide. The metal you fold is the bottom of the trim kit, which should fit neatly into the cutout portion of your countertop.
Create a seam along the cut corners of the metal that you just bent with a welder. Welding the corners you cut in Step 6 will give the trim kit a nicer appearance, reinforce your work and prevent accidental cuts.
Using a sheet metal folding tool or a locking vise grip for sheet metal, fold down and round the outermost parts of the trim kit so the raw edges are not exposed. Fold the metal back from between 1/8 of an inch to 1/4 of an inch.