This is a "plate" that you can mount a raspberry pi 2 / 3 to, add some M3 x 25mm+6mm Male to Female Standoffs and another plate and stack them until you run out of raspberry pi's, or hit the ceiling. There is a space for an optional cooling fan.

Additional parts needed:

I toyed with printing these, but they just were not as sturdy as I wanted them to be. The posts were fine, but being able to stack them required additional hardware, or a small printed peg, that just didn't hold up to what I wanted.

• M3 x 25mm+6mm Male to Female Standoffs
• 4 M3 x 5mm screws for attaching the standoffs to the plate.
• 4 M3 nuts for the top to hold down the top plate
• M2 standoffs for mounting the pi to the plate. Here is a kit that will have all the parts you'd need for this.

Optional:

30mmx30mm fan and screws to mount it