don't worry about that. inserts come in set depths. I think for mine it was 18-20" or so? the cabs are 14" I believe, so depending on where your hood is placed, you can go from there.

I have a very low ceiling so I had to work w/what I had.

the insert is 34-35", and the builders built the hood for it. (insert will always be an inch smaller than the actual hood. 36" for a 36" stove top.

I got a piece of walnut, stained it, and had them run it around the bottom

if you don't have a cab, it will look something like this:

as you can see, they all protrude

Usually the cabinet maker just builds the hood a little larger inside so there’s some room to swap inserts later. As long as the width is standard you normally won’t have much trouble finding another one down the road

Or decide you don't want to look like an industrial kitchen and skip it all together. Truth is this is not a thing. A reputable dealer will have parts. If you don't trust they will still be around, buy an extra fan now. It would be HIGHLY unusual for it to fail in your lifetime.

For lowest noise, an external or in-line blower should be used with an intermediate silencer such as Fantech makes. In-line and external down-slope blowers are available from Broan/NuTone, and modest CFM down-blast blowers from Captive-Aire.

I have a nominal 600 CFM down-blast NuTone blower acquired in the late 1980s still operating, now as a blower for ceiling ventilation over wall ovens.

With this scheme one uses a wall or cabinet mounted motor control (like a rotary light dimmer but the functionality is off to full on, then speed reduction).

The insert then is purely metal and if made of an appropriate stainless steel is essentially a lifetime of residence unit. See Wolf for examples. Blower or control failures are independent of the insert.

I personally have a cab9net finishes wood hood. I also have put numerous wood hoods in clients homes. I have never had a complaint that they are greasy or get dirty. I have however had a client who originally wanted a stainless steel hood come back and ask if there was a way we could modify it because the stainless steel was and I quote " a &^%$ to clean" . Get what you want. Don't worry about future inserts. They are standardized, there are just a lot of options.

If a hood's entry aperture size is sufficient to overlap the rising and expanding cooking plumes, and the specific air flow rate (CFM/sq.ft.) is sufficient to avoid plume spillage (internal reflection of the plume momentum), there shouldn't be any grease on the exterior of the hood other than that which migrated there from opening oven doors when broiling or roasting -- in other words, the same level of grease found on the walls and ceiling.

Schlieren photo illustrating spillage