I would also be somewhat concerned about operating what looks like a 1000-1200 watt microwave
in such a confined space. While it may not start a fire in it's own right, I really doubt if the internal electronics and the magnatron will like the extra heat build up. Built in or under cabinet types usually have a front vent. And what does the manufacturer say about clearances? My home units always say something like "Do Not Use in a Confined Space, Such as a Cabinet" or "This unit is not designed for built in use" etc.
As an aside: We built microwaves into several RV's in the past and found that basically, because we didn't have a hook up, or just didn't want popcorn that night, we seldom used them, so stopped the practice. But each of us is different.