I’m confused. In one picture it is open above the microwave, an another there appears to be a cabinet box there? I see two solutions. If you want to keep them open, build a false wall above the upper cabinets just deep enough to hide the pipe- meaning the cabinets are 12” deep but above the back wall there will only be 6” from the face of the wall to the cabinet. Sounds goofy but it disappears in reality. What I would really do however is affix risers to the cabinet boxes so all of them extend to the ceiling then add crown.

If this is vented to the outside why did you go with a MW above the range they are so so at best for venting and awkward to use when the range is in use. Not sure who designed this kitchen but quite a few problems beyond the MW issue. One being the huge fridge sticking out into a tiny space

So to answer questions back from you all...

Re photos - in the top photo you see the empty space where the microwave should be installed, and the upper cabinets above it that were supposed to house the vent/duct into the wall to run outside. The contractor ran the venting to come in from the ceiling instead of into the back of the upper cabinets above the microwave. The second photo was one attempt at a solution- to stack a 2nd cabinet on top of the uppers. Not a great fix in my opinion.

The microwave is existing and was less than 1 y/o so no need to replace it. This is a remodel not a new build and all the electrical etc was set up there for a stove/microwave combo

I guess short of buying another upper cabinet w/ different dimensions, or custom building some sort of surround for the vent, I don't really have any other ideas. But at least you guys validated what I had already researched as options. Thanks.

You could add a stack above the cabs to hide the vent, but you'd have to move it closer to the center.

I'd still sell the microwave and put a real hood in there, but that's just me.

You need a skilled installer or hvac guy to do this: take the rectangular ductwork and turn it to the left and go outside (exterior wall0. Do this ON TOP of the cabinets. Then get a large enough top crown molding to hide it. You'll need someone who's good with ductwork but this is doable. We've done this before in the past when we've had to get creative.

We have an OTR microwave and it is not vented outside. Our microwave can be vented outside if you want or fixed so that the steam comes out through the louvers on top of the microwave. We live in an older home and it is easier for us to have it this way. As for your problem...I would just box the pipe in and it will look okay.

What tkp is saying above is what we did in the attached kitchen- stove is on an inside wall so the duct runs horizontally above the cabinets to the outside wall. A false wall is in front of it and the crown distracts the eye from the fact that the wall above and below the cabinets are on different planes.

I'd just box the vent pipe in with a metal box between the cabinet and the ceiling, as shallow as possible, centered horizontally on the cabinet but just wide enough to cover the the pipe. Attach the box to the top of the cabinet and paint it white or the color of the wall. Sure, you'll know it's there but after a while you won't notice it anymore. -- amateur