A set of connectors for making boxes out of 20 mm square aluminium profiles

This is a set of 3D-printable connectors for making heavy duty boxes out of 20 mm square aluminium profiles. The pictures shows how I used them, but they are universal for any size of box, just cut the aluminium profiles to the desired length. There are connectors for just a box, for a box with intermediate brackets, with mounts for 3D-printed wheels and with mounts for a trolley-type extendable handle.
Instructions:
* For normal corners use corner.stl. 8 of these will make a simple box.
* Fo