Single Raspberry PI case or stack them as high as you want. PI 2,3,4,5 will fit the cases. Use a fan hat on each PI for proper cooling. Use a M3 screw in the back to hold the PIs in place. Print as shown to avoid supports.
Pi 5 Case
Metal Case for Raspberry Pi 5, Supports Pi 5 Active Cooler, Hats, Add-on Boards, NVMe Hats (Black)
Pi 4 Case
Raspberry Pi 4 Fan ABS Case with Cooling Fan, Heatsink, Simple Removable Top Cover for Pi 4 Model B/ 4B
Pi 3 Case
Raspberry Pi 3 Case, iUniker Pi 3 b Case, Raspberry Pi 3 Power Supply with Switch Raspberry Pi Power Supply
3D Printed Case for PI 3, 4, 5
Free 3D print Simple Multi-PI Case that fits the PI 3,4 or 5