This is a minimal yet relatively secure way to stack your Raspberry Pi's vertically. This is a universal B design in that it works with the original B, the 2 B, and presumably the B+ (I don't have a B+). The design goals were something that was quick, minimal, and held the Pi in place even when cable tension wants to pull it away.
- Print one "sled" for each Pi.
- Print one "pi topper" (not really needed, but makes it look better).
- Remove the SD card from each Pi.
- Insert the Pi with the SD card slot facing the rear (the rear is the part with the larger wall and missing a bottom support).
- Gently push each corner of the Pi down past the notches. This may take a little bit of flexing of the printed part but ABS didn't require much work.
- Re-insert the SD card.
- When stacking the sleds, align the supports and holes and push into place. This is a friction hold, but it has held great for me.
- Put the topper on top.
- Curse the placement of the components as you try to plug the cables back in.
Yes, the last picture is upside down just to show that each Pi is secure.