Putting a small microwave in a cupboard where it will fit snugly and not take up too much space may seem like a good idea, but it may not be the best solution.
Microwave ovens, even the smallest ones, have an exhaust system that allows them to control the temperature and eliminate excess hot air or steam.
Thus, putting a microwave in an enclosed space will only cause the air inside to become stuffy and the entire area to become moist after a while. Eventually, mold will grow in the cupboard, causing it to smell and become unsanitary.
When it comes to finding a place for your microwave, it’s best to just place it somewhere on the countertop, where it won’t take up too much space. This can be a surface (or something else – the important thing is that it is not enclosed)