Using https://www.thingiverse.com/thing:2536972 Corners,
we are now going to build a 20' tank container.
You always need the same amount of Corner and Corner_mirror, because the oval
hole in the corner should be longside in the upper and lower position of the container.
Edit 2018-07-23: Added thicker connection ring, 3mm, for those who want it.
Watch video for assembly.