Make an angled template, take the piece to a local metal fabricator, and have the angle cut.

Problem with that is the bottom piece mounts to the range hood then a bracket at the top. The inner extension also mounts the same way with a bracket mounted to the wall at the top and two set screws on the right and left to hold it. Cutting the top piece at the correct angle would go trough one of the 2 louvered vents. I know I would have to drill a new mounting hole also.

Why are there vents in the chimney at all this should be a solid piece that goes to the ceiling. Maybe a pic from further back of the hood and the chimney

I believe it is just a decor thing. The actual venting is through round expandable vent to roof inside of this piece.

Those holes in the side are if you want to convert it to a recirculating hood. Not decorative at all.

Maybe get a pro involved with this before you really mess it up.