Quote: "How you managed it I have no idea"
Faith, Trust and Pixie Dust.
But, seriously, I don't do anything special. I just make the floor panel a perfect square and then, position the walls (in Blender) hanging halfway over the edge of the floor panel, but, positioned on top of the floor.
See attached image as to what I mean. Please excuse the terrible art, just a quick doodle to portray what I mean.
Cheers
Jay.