If one thinks of a island hood hanging from a ceiling (actually from the joists above a ceiling) via a chimney (duct cover, hood extension), one may appreciate that the cover itself is performing the support, and that a flange on the inside of the chimney is used to provide the structure bringing the hood weight load to the fasteners and thence to the joists. In the case of my Wolf hood extension, there is an internal flange at both top and bottom. The upper attaches to the extension to the ceiling; the lower attaches the hood to the extension.

It follows then that for this structural approach that the chimney has to either be fabricated to the exact height needed, or a section removed and the remainder welded together, with the welds being of the same class of quality as those of the hood itself.

This is a task for a sheet metal shop. They can best decide whether to modify an existing chimney or fabricate a replacement. I note the latter because with material of the same gauge as the stock chimney, the flanges can be bent from the flat stock, and the rectangular shape of the chimney then formed, leaving only one edge to be welded.

It may be possible to get a hood manufacturer, local or maybe even Modernaire, to make the desired chimney. But an example would have to be provided, or an excellent drawing.

In the alternative case that I have never seen, there could be an internal structure that caries the hood weight, with a thin chimney skin somehow attached. In that case two parts would have to be modified.