This a case for the Raspberry Pi 4 that allows you to stack an arbitrary number of cases on top of each other for cluster raspberry pi builds. The thumbnail shows 3 but go as high as you like. Each middle clips into the ones above and below. When you are done stacking, add a top and bottom to the stack. To be clear: You print out as many middles as you want, and cap the stack with a single top and bottom.
EDIT1: Reuploaded with additional STL for a bottom lifter that allows air flow to the bottom Pi. Renamed the main stacking module to "middle." Credit to user IGV2016 for suggesting the addition of a bottom to the stack to improve air flow.