One thing I might suggest: You don't need the vent parts to be cut perfectly accurately. It's always good to make the cuts as accurate as possible, certainly, but in this case nothing shows but the inside. So the important cuts are the ones that set the height, width and depth of the inside of the duct. Pay attention to those. But otherwise, one part can overhang the other inside the cabinet just a little bit. No one would ever know.

Use butt joints so perfect alignment doesn't matter. Pick one dimension to make perfect (height or width) and measure and cut it carefully. If you miss, do it again until you have an exact fit on those. Then cut the others close, but don't worry about it if you're a smidge off. They may overhang 1/16th inch or maybe that much too small. Just line them up on the baffle, apply a line of glue and clamp them in place. When you're done, you'll have a perfectly flush fit on the inside edges that show, maybe on the outside too but if not, no big deal.