Super Simple 180MM Quadcopter Frame
ATTN: there is a new version of this model here: http://www.thingiverse.com/thing:955440
For absolute minimalism you may prefer this one though.
A first try at modeling a 180MM frame for a quadcopter. The only holes modeled into it are for the motors (12mm crosswise) and PDB/Flight controller (30mm edgewise).
It surprised me with how well it flies.