This is a sturdy 3-sided corner bracket with Hexagon sides and 4mm thickness.

Each side is 64mm long (with ~85mm hypotenuse) and has 3x M4 holes to mount side walls 

(e.g. for a 3D printer enclosure).

Hints for the print:

The Hexagons can be oriented with the sharp corners pointing up, to make it print nicely (see attached gcode).