Ruins
I like corrugated cardboard, because it is cheap (I usually get it for free) and because it is relatively stable. Some of my favourite terrainpieces are made of corrugated cardboard.

This ruin is made from corrugated cardboard. I have decided to make two corners - one large and one small. The small corner will be placed opposite the larger one, creating the illusion of a ruined building.
As can be seen in this picture, I am cutting out the doors and windows of the ruin now - before adding the second floor, the ruined inner walls and the stairs.

In this picture you can see the ruin after the addition of the second floor, the inner walls and the opposing corner. I am experimenting with methods of closing the corrugated ends, especially in the windows.
Ok, at this point in time, I am almost done with painting the ruin. I have used Polyfilla to fill the corrugated ends, but I have to agree with Gary James - the Polyfilla has a tendency to fall out once it is dry. A better idea is to mix sawdust with white glue and then use that to fill the gaps. Before painting, I covered the ruin in white glue and flocked it with sand to create a suitable surface for drybrushing. Unfortunately, the sand was too coarse and I was too liberal with the application of white glue, ending up with a sandy building that had lost some of it's details.
About the painting:
I have choosen to use two shades of blue for the primary color scheme to make it match the Necromunda look. The darker blue has been painted on and then I have been drybrushing the lighter blue on. If I have the energy, I might end up drybrushing with white as well. But trust me - drybrushing this building hurt my arms badly!!!

This is the dark blue first layer of paint. Notice the plastic girders (white sticks sticking out of the floor) - that is actually the end of a plastic tea-stirrer.

A very bright look of the inside of my ruin. Notice the rubble - cat litter.

Now the ruin is almost done. I've picked out the doorway, the windowframes (cardboard) and the spikey decorations with the very lightest blue and a bit of white. I've also painted the girders black. I should weather them with grey drybrushing. I've tried to make the floor look like dirty concrete, but it is not really convincing.
Learning curve:
I have learned from this building...