Metal fabricator.

Look at the sheet aluminum stock available at Lowes. Without a brake, you’re not going to be able to get sharp bends, but creating an L shape in aluminum is pretty easy to do, bending it over a 1x4. Drill the holes for securing to the studs in the wall before bending. File all the edges so there are no burrs.

It’s all about your measuring skill to make work. A fabricator could do a J shape in stainless that would hook under the back edge if the hood and leave it easier to clean. It will be a cleaner straighter bend, and look better, and stay looking better over time. And won’t be that expensive.