I have a huge supply of 1cm square pieces of 5mm foamcore which are the cut-outs from the windows of my desert village and other projects. I've used some of these to form the stacks of crates.
The tarpaulins are PVA-soaked kitchen roll draped over the foamcore stacks. The depot staff have used large wooden poles to weigh down the edges of the tarpaulins, these were made from bits of barbecue skewers. Everything has been painted using Inscribe acrylics.
I've tried to make sure that the steps in the crate stacks are large enough for my figure bases to stand on (eg. GZG armed colonist and Rebel Minis contractor and gang member).
The little hut is a model railway building. Being designed for HO/OO scale it's a bit large when compared to an average height 15mm figure (a GZG colonist in this case).
I solved the problem by creating a raised concrete step in which the hut sits. It's just more foamcore scraps, I also shaved off the original lower door hineg and made a new one out of paper (it even has blobbed-on PVA screw-heads, though they're hard to see in this photograph!).