I use adapted lenses and in-camera movements with most of my photography and OEM lens hoods often restrict the size of the image circle projected by the lenses, so I have shortened quite a few of them over the years.
Most recently, I shortened the OEM hood for the Sigma Art 14-35/f2 zoom to remove the petals -- see the photo below -- and if you're even slightly handy, it's not very difficult at all.
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Plastic hoods are easy to work with than metal hoods, but the principles are the same for both: Carefully wrap a strip of masking tape around the lens to indicate the desired cut line, then use a hacksaw to cut very close to (but not right against) the tape. This is because you'll want to leave yourself some room to sand the material smooth without shortening it too much.
Of course, you'll have to figure out a way to clamp the hood against the work surface or, worst case, get someone to hold it for you and I recommend cutting it only partially through and rotating it several times rather than cutting it all the way through in one position.
Once that's done, you sand the rough edge smooth, either by rubbing the cut edge in a figure-eight pattern on a piece of sandpaper that's held flat against a piece of glass or another flat surface, rotating the hood in you hand occasionally so you sand it evenly. (Or if you have access to one, a bench-mounted disc sander works well for this, too, but go slowly because it can remove material very
Start the sanding process using a fairly aggressive grit -- say, 80 or 100 -- to do most of the smoothing, then use a couple of progressively finer grits -- 220 to 400 then, say, a few final passes with 600 to 800 -- to make it as smooth to the touch as you wish.
Finally, use a small piece of sandpaper in your fingers to rub around the sides of the cut edge to smooth those over as well. If the hood is plastic, then you're done, but if it's metal, you may wish to paint the exposed edge or at least run a Sharpie around it a few times to make it black.
That said, though, the fact that you felt the need to ask this question does suggest that maybe you're not the DIY type -- no offense intended! -- in which case, you might ask a local machine shop to tackle it for you (pretty much any place should be able to do it, including even a general automotive machine shop) or send it to S.K. Grimes -- http://skgrimes.com/
-- or another camera machinist and have them do it for you.