The fractal Terra is small but not stackable; easy access to everything but sata drives. In the past I used a Sugo 13 but the psu takes up most of the space.

Lian li and Asus have some good micro atx options like the tu150 or the ap201. Overall small compact machines that stack are best suited to 1U/2U rackmounts in my experience

they also have tons of them just laying around, gives them a really powerful, efficient display controller.