" My upper kitchen cabinets are 15” deep. Under cabinet exhaust fans are not this deep"

Perhaps I am not understanding you. Undercabinet exhaust hoods - even recirculating ones - are usually 18" deep or more front to back. The better ones are 24" deep, and you can find many that are 22" deep. Rather than fooling around with wrapping in sheet metal, etc., get a deeper hood.

The cooktop installation instructions will specify distances to combustibles. These values do not exclude combustibles that have sheet metal attached to them, unless spaced out by appropriate methods (see IMC 308.4). So any sheet metal used to block cooking plumes headed for the gap will have to be appropriately mounted if within the combustible distance limit. This limit varies greatly with type of cooktop and burner BTUh.

A small under-cabinet exhaust fan (blower) that doesn't exhaust must filter well to have any utility at all. I'm not particularly sanguine about what filtering can be fitted to such a small unit as the OP describes.

It should also be understood that small flow rate devices cannot adequately capture and contain hot oil or grease cooking plumes, which will spill out into the room.

Go to a local sheet metal shop with the exact design & dimensions you want and have them bend it out of stainless steel.

I don’t fry foods so the oils are not an issue. I am fully aware that charcoal filters need to be used. To the person who states these units are all 21 inches. This one is only 18” deep. Not all of any of these under cabinet units go UNDER the cabinet, there is always about 6 inches overall in front of the cabinet. In this case that creates a 5 inch wide void. It will likely be easier to fill a 5” gap over a 3” gap.

I contacted 3 sheet metal shops, which is all we have in this county. One does small jobs; waiting for designer to contact me while I look outside my area. Thank you all for the comments.