I want to echo Weed's caution about being sure on code compliance when having your vent going out near that window. Last year, when I was doing a re-routed of the vent on my range hood, the local building inspector happened by. (We are on pretty good terms so there was no compliance issue.) I asked him about the rules and got a long dissertation as he handed vent tubing up to me. The gist of it was that the rules for range hood vents are different than for those that directly vent gas appliances (say a water heater power vent). There is some discretion when it comes to venting from a range hood over a gas stove. How far the vent has to be from an opening window also varies with whether the vent is above the opening window, at window level or below window level. I was told the vent that is above an opening window can be closer than one at or below window level. BUT, you want to be sure that, if you open the window while the hood is running, it won't be sucking in the vaporous stuff that was just blown out the vent.

You could do a vent hood enclosed in cabinetry. Chemetal makes copper laminate, perhaps you could design some kind of enclosure that used copper laminate as one of the materials. Here is a link that might be useful: Power packs for custom hoods.

A custom duct cover is probably less expensive than getting an island hood to solve your problem. (The "chimney" on the hood is really a cover for a duct that the air flows through.) Because an island hood has to be finished on all 4 sides and has to support its weight from the ceiling, it normally costs more than a wall hood which normally has 3 finished sides and is supported from the wall. About the smallest price difference I've seen between the wall and island models of the same hood was $500 and it can be over $1000. The custom duct cover for our hood cost about $500. That was to get a duct cover made with trapezoidal sides because there was a sistered joist in the way of where we wanted to hang our island hood. Since it was for an island hood, that was for something strong enough to support the hood. That is a lot more weird request then getting a "flange" on a wall duct cover. A wall duct cover usually has 3 sides with just flanges at the back for screws. So what you want might be more like a small step or notch in the sides of the duct cover so that the sides are 1/4 inch deeper above the backsplash. I'd expect that to add much less than $500 to the duct cover cost. To ensure that you get exactly what you want, you should draw up a sketch with dimensions showing it. Independent, who made our duct cover, required a sketch from us before they would quote. There might be some practical problems with an island hood against a wall. If it is installed tight to the wall (so that its rear edge touches the backsplash, there might be noise from vibration since it won't be screwed to the wall like a wall hood would. If it isn't tight to the wall, it might be difficult to clean behind and some grease and steam laden air might flow behind it. If your backsplash goes behind the hood, screws will need to go through it that attach the hood to the wall.

building2017 wrote: ... - What are the silencers I have read about and how do they work/would they work here? These are like giant truck mufflers, except designed for various diameter ducts. They have low pressure loss (forget the value but it is a lot less than baffles). Fantech is the source. - Any benefit re:sound to using two smaller liners instead of one big one and only using one at a time when using only one side of stove? Or is this magnifying the cost for no good reason other than sound? (Because I'm assuming I'd then have two runs up and out and two things on the roof because they wouldn't combine to avoid a turn and to ensure proper function? If only one side were run at a time, then MUA costs could be lower, but where code requires MUA, I am willing to bet a donut that the AHJ will demand an MUA system that can handle both at once. In theory, the sound reduction will be only 3 dB less. Also, when using the central burners, you will still need both systems for capture and containment. Well, I guess three things on the roof then with the MUA. While commercial systems mostly use roof MUA entry, and I use roof MUA entry, the options for residences are wide open and may include side of house entry, or entry into a basement that has an air path access to the kitchen.