Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4923288.

Stack as many raspberry pi 2, 3, 4 as you like. Use 2.5mm X 6mm standoffs for the board and 3mm X 25mm standoffs to stack the boards. Use standard 30mm fan and hardware. The top board should have a guard over the fan.

Heatsinks are required for this design

The project is on Tinkercad if you need to remix...

https://www.tinkercad.com/things/0cPbfxi2EyN