I would get fabricated stainless for the bottom.

The capture aperture has to overlap the cooking pans by enough to collect the expanding cooking plumes. If you must use an undersized assembly, then it should be high with a sheet metal angled "skirt" effectively extending its capture area to the needed capture area. The blower should be sized to the new aperture, but I see that the unit comes with a blower that is too low in CFM for that duty. So hot oil and grease cooking (wok cooking or meat searing) will probably not be fully contained, even with an expanded "skirt."

Review your stove's installation instructions w.r.t. combustibles. Distance to combustibles means by any direct path in the specified direction, even through non-combustibles unless protected per IMC Table 408.4.2. In other words, if the required vertical distance is 32 inches, and the sheet metal is at 30 inches, but the internal 2x4 is at 30.1 inches, you risk deterioration of the wood making it more flammable (and weaker) in the long term. I believe "drywall" is considered a combustible, even if of the fire resistant variety. You should review your final "plan" with the local code inspector before expending effort in cutting materials.

And your photo island trim stove is likely to be in violation unless the wall is brick all the way though, or built with steel studs and non-combustible surfaces (both sides), or the installation instructions allow island trim against a combustible wall.

"The insert is only 19 inches wide and the full hood cover will be 30 inches wide."

You need to get a new wider insert. I am not trying to be funny or snarky, I mean it. What are your reasons for having a 19" wide insert over a 30" cooktop?

The insert is designed for a 30” cooktop! But I realized that Broan sells an optional 30 inch metal liner to insert it into so I’ve ordered that. Thanks for the responses though!

Hint: it is not ”optional”. Everything is a la carte in the ventilation jungle. And it is still insufficient for the job, even with adding a liner to protect the wood. You just won’t scrub the finish off of the wood trying to clean it. Youll still have grease escape and go elsewhere, all over your cabinets.

I assume you mean 19" DEEP? How WIDE is it? For typical 30" range you'd want a hood with an opening of 36" wide by at least 24" deep, ideally deeper.

More: https://bamasotan.us/range-exhaust-hood-faq/

"For typical 30" range you'd want a hood with an opening of 36" wide by at least 24" deep, ideally deeper."

That isn't even possible unless you have no upper cabinets. Obviously that is the case for the OP, but most folks I know have a range hood that is exactly the width of the range. The upper cabinets don't allow for anything wider.

Well some kitchens are designed with the upper cabinet opening 6" wider than the range to accommodate. The 24" depth is what I would think is unusual for a non pro range or cooktop. Your head would be underneath a 24" or deeper range hood.

The canopy of the range hood is not coincident with the workings of the range hood. My cheapo range hood, which will be replaced when the kitchen is renovated has a motor and screen that is about 12" wide and deep in a 36" wide range hood. And that is probably what is in the 24" wide and 30" wide hoods. The inserts are smaller because they are just the guts and are meant to be put into s larger canopy, because a certain amount of that big hood is not motor or fan it's empty real estate.

I've not seen upper cabs with that wider opening, at least not in any that I have been in. And yes, 24" deep is a bit wacko. Mine is 19" deep - definitely would not want it sticking out more than that!!

It's pretty commonly done in kitchens designed in here, extra inches on each side. And the house I grew up in had a wider hood than cooktop. But the actual screens and such were not 42" wide, there were lights and empty space inside the canopy. It was a Thermador hood.

I don't really go into houses that have "designed kitchens" I guess. Just regular older homes, nothing fancy. Even my sister's brand new home (well, 2 years ago) does not have the wider upper cabinets. And my brothers almost-new retirement home does not have that either.

It’s been a venting design best practice for over 75 years. Even my mother’s original 1952 kitchen had a 36” vent over a 30” cooktop. Lots of ventilation failures out there, as your nose can tell you if you ever go into homes with a love for cabbage or stir fry. Rancid grease and cruciferous vegetables aged on wallpaper and upholstery is not a pleasant thing.

75 years - ha ha.

Really. I have no "ventilation failure" LOL. I have an excellent hood, and it does an excellent job.

Well, aren’t you just glad that you can thumb your nose at all of the professionals and be nose blind about your own house.

"nose blind"? That's hilarious. Honestly some of you folks here are so so full of yourselves.