I don’t understand the issue. You need a bigger mounting plate? You need a different cap type? This doesn’t make sense.

If the issue is that the dryer vent is aluminum and kitchen cooktop venting requires steel, then the issues are: (a) potential increased hazard if there is a duct grease fire -- something that the hood baffles or mesh filter should help avoid; (b) a code enforcement inspector requiring that it be replaced at some inconvenience and cost. I have an external blower, and all of them that I have seen have a flange for shingle overlap. This would do for siding overlap. Without a photo of the approved cap, I cannot determine what the interface problem is.

By the way, for best resistance to wind, there should be a damper at the hood and one at the cap.

You are using a "Plastic" dryer vent? That doesn't sound safe.

Clarification: I got the correct metal cap, but there are no mounting blocks that accommodate it, so they usually just get surface mounted and caulked to death. Hubby wants to be able to tuck it inside the siding for a better seal, so he got a different one - not to code - that he insists on using instead. He and our carpenter just retired form the fire department and they went to far more dryer fires than kitchen fires - and those were usually calls about people freaking out about their ovens self-clean mode, lol. Yes, I’m also a bit concerned about the eventual inspection ding. But I don’t think in this case it’ll be a big deal to change then. I’m just frustrated with his insistence on improvising like this.

Yes, the cap does have a 1” flange, but it would have to come out at just the right spot to be able to lap the siding over it. All the installation videos I’ve seen so far have them just shoving it up flush and caulking. Maybe we could just mount it flush to the Buffalo board and add a little more flashing, then carefully trim the siding up to it? Maybe box it in with J-channel? I understand why he wants to make sure it’s weatherproofed.

So the carpenter has been to more dryer fires and is putting a dryer vent on a vent from a kitchen hood that vents heat and grease?

Here's our vent on the roof. It is metal, has a damper and has a flange which slides under shingles. Surely they make other metal vents like this with flanges that would work with siding?

You need to go to a real HVAC supply house. Not a box store. You both have the wrong product in mind.

Perhaps. There is another firefighter friend who is also a plumbing and HVAC sensei, so maybe he can get access to a better cap. Here is the steel one I got https://www.homedepot.com/p/Speedi-Products-6-in-Round-Galvanized-Wall-Vent-with-Spring-Return-Damper-SM-RWVD-6/202907285

On a side note, we are so blessed to know all our firefighter friends. These guys are all in different professions - both white & blue collar. We have nearly every trade represented on the FD! They’d be good friends regardless, but it is a relief to know these guys personally. This HVAC guy is now retired from both jobs, but he was the maintenance engineer at a major university here in Mpls, and his soldering skills are unmatched! He helped us take off all the old valves in the laundry room and half bath to make tiling easier and just to update them. Unfortunately, the caps they used didn’t come off when it came time to put the new valves on. Apparently they had some brass in them. IDK. So they had to cut them off and make the copper even shorter. The new floor was thicker and a couple of them were already really close. His plan was to put compression fitting on, but those two that were short were impossible. He finally decided to solder them, but he was working inside the tile! The hole was small and he was really concerned. We were dangerously close to having to rip out the basement ceiling. He managed to get them both on perfectly and there was a collective sigh of relief! He’s got skills! I think he deserves dessert when we take them out for dinner, ha!

I missed the plastic mention.

In any case, you have dropped the duct diameter by two inches which will lower achieved CFM, and further want to use a cap that is significantly even more restrictive. Note that the roof cap illustrated by @vinmarks has a reasonable 'aperture' for the air to get out of. The home depot version does not. See Broan, source of your hood if I recall, for a proper cap.

The ducting for our hood is 12 inches with a 12 inch cap. We ordered it from Vent-a Hood. They do make wall louvers so you could check out their site to see if one of their wall louvers would work for you.

https://www.ventahood.com/index.php/products/venting

The whole soldering kludge saga lends a lot less confidence in the on board abilities. Not more. Doing it wrong successfully does take skill. Doing it right takes more knowledge, and gets the better result.

There’s a whole lot of planning devoted here to doing it wrong, rather than doing it right.

Keep your snark to yourself and piss off

Get a metal exhaust vent here:

https://www.supplyhouse.com/Wall-Vents-24275000

Those aren't suitable. A wall cap providing the functions of this example is needed.

https://www.broan-nutone.com/en-us/accessory/843bl

Regarding my post about: Get a metal exhaust vent here:

https://www.supplyhouse.com/Wall-Vents-24275000

Click on the aluminum and galvanized steel subchoices - that didn't carry with my post!

Alright, so hubs finally relented about the vinyl dryer type vent and we're back to using the galvanized cap w/ damper, but the mounting still seems screwy just sitting on the siding like that with caulk to seal it. Yet, thats what the pros from the local HVAC suppliers have all confirmed. Weird. Another option would be to transition to the long rectangular vent and Broan sells wall extensions for those. I'll keep checking around.

Thanks all.