Are you trying to connect to a vent that is in the wall behind the MW? What is the model of MW you are trying to install?

If the attic is right above the MW, then venting straight up is the best option. Cutting a hole in the drywall ceiling and installing a sheet-metal vent is a very straightforward job, as long as there is no ceiling joist right above the area you want to cut.

Bruce

Don’t do an OTR. Do an actual hood and put the MW somewhere else.

I assume you have a wall vent you can do something like this... Most HVAC contractors can make this hook-up and if you DIY and you handy, you can do this yourself.

Agree with Sophie. That bump out of the cabinetry over the range for the microwave would impede cooking IMO. I know I would bump my head on the microwave with it sticking out that much further (6" additional, right?). I don't know what your kitchen looks like, but there are numerous other better functioning locations than OTR. Here are a few ideas:

^next to the range but not blocking use or access to the entire range top

^In its own quick prep station away from the main cooking areas of the kitchen

It depends on the size of the kitchen. Tiny kitchens benefit from an OTR microwave.