Microwaves need airflow. If the manufacturer made the trim kit with vents it’s because the microwave needs the airflow.

I agree with fidlfreek. The trim kit has vents in it for a reason.

How important is this to you? You can certainly custom-build a solution that works and that doesn't have vents. But it'll cost you.

For one, you need to find some other way to vent excess heat. Heat kill appliances really quick.

This would most likely require modifying the back of your cabinet and installing a vent chase. You also have to redirect airflow from the front to the back. That probably requires installing a fan that turns on when necessary. A fan controller with a temperature probe might do the job. You can get those for a few dollars from Chinese websites. If that doesn't work, then you could also install a current-sensing controller that turns on whenever the microwave operates; but that might be another custom-job to build the electronics.

For the trim kit, you could just go to a local metal-fabricator and ask them to cut a piece of stainless steel with a water jet. You'd have to provide exact drawings specifying the precise dimensions. And you'd have to figure out your own solution how to install it into the cabinet.

I have done things like this on occasion, when I wanted to use unusual appliances in very specific locations. Results can look great. But it requires a lot of work and costs are high. I think this would be overkill for what you are trying to achieve. I'd go with the factory-made trim kit.

It's a counter top microwave, get a built in designed for a cabinet, or another for use both ways with a plainer trim. There are a zillion There is nothing I can see so special about that one