This is a corner bracket for the MBeam thing.
To build a square you need 12x MBeam and 8x brackets.

The top layer needs bridging, its a bit odd sometimes (using slic3r)
I printed with PLA (190C) and 10% Infill.

Tip: Try out GCodeSimulator to check you print upfront
Homepage: http://3dprintapps.de
On Thingiverse: http://www.thingiverse.com/thing:44286