There are a lot of NAS case you can find in China(Taobao), they are cheap(usually $40 for case with 4 hotswappable 3.4 drive. They use mini ITX motherboard and

There are a lot of NAS case you can find in China(Taobao), they are cheap(usually $40 for case with 4 hotswappable 3.4 drive. They use mini ITX motherboard and flex 1U PSU