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.

To use:

1. Print one "sled" for each Pi.
2. Print one "pi topper" (not really needed, but makes it look better).
3. Remove the SD card from each Pi.
4. 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).
5. 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.
6. Re-insert the SD card.
7. 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.
8. Put the topper on top.
9. Curse the placement of the components as you try to plug the cables back in.
10. Enjoy!

Yes, the last picture is upside down just to show that each Pi is secure.