Just a top, bottom and middle standoff for stacking Raspberry Pi's, hats, shields etc. Thanks to http://www.thingiverse.com/thing:1477398 for the initial inspiration.

I made the pins longer so that they'll protrude farther through the hat, and more firmly seat into the next in the series. Mix and match combinations of base pins, "nuts" and standoffs to get the height you're looking for.