I have just spent half the morning de-stressing myself... and I feel so much better now.
This was knocked together from 2'x1' of 6mm MDF and a few scraps of thinner stuff for the floors. The base is a 1'x1' square, the walls are cut from the other square foot. It stands just over 9" high, with floors at 3" and 6". All the windows were cut out with a jigsaw on the workmate.
Once all the glue has dried, I will tile the floors, add a bit of demolition wreckage to the ground, and spray grey. Easy, sturdy, usable.
And I feel relaxed for the weekend...