There are cases for use without GPUs with standard desktop components, like In-Win Chopin.They are like 3/4 liters, so still really small. Comes with a tiny in

There are cases for use without GPUs with standard desktop components, like In-Win Chopin.They are like 3/4 liters, so still really small. Comes with a tiny inbuilt 150W poeer supply. Fitted it with i3-8300 cpu and it runs for years. You can get even thinner cases if you use slim motherboards with DC barrel jack input