This post-apocalyptic shanty is just a box made from cardboard: ordinary corrugated box cardboard. Glue two layers of card together with the corrugations at right angles to prevent warping. To make the metal roof, you have to peel corrugated card so that the corrugations show (or use the single-layer corrugated card you sometimes get in pizza boxes). The grated window is aluminium auto-body mesh surrounded by snapped-off craft sticks, and the wooden door (which is absurdly huge; needs a lintel) is coffee stirrers from McDonald's. The texturing on the walls is just Poundland polyfiller mixed with PVA glue; waiting for that to dry is the most time-consuming part of the whole process. Figures from em-4.
Intended as a tunnel entrance for Necromunda, this is all just cardboard, although the base may be a scrap bit of hardboard; not sure about that one. The "corrugated tin" surface reappears to make the curve easy. The disproportionately huge rivets are stick-on gems used for scrapbooking or card-making; you can get a bazillion of them at The Works or Hobbycraft or wherever for a pound or two, and they're great for greebling. Card panels cover all the joins to make it look a little neater than it is. Figures from em-4 (on the right) and Copplestone (on the left). If you wanted to make the door look nicer you could print out a sign from the internet and stick it on.
The legs of this watchtower are till rolls (one of them topped with part of a candy container), and the rest is just scrap card, cocktail sticks, more corrugated card and a bit of scrap hardboard for the base. The ladder is from the Lord of the Rings minis game and came with a load of minis I got on Freecycle. Obviously, the different walls of the thing are completely impractical, but I wanted it to give different levels of protection on different sides to make it more tactically interesting. The bottom figure I'm pretty sure is Copplestone but I don't know where the upper one is from; I got him second-hand and he's absolutely huge.
I am not saying my terrain is the best-looking, but making it is fun, it's quick once you get the hang of it, and all three of these pieces put together cost me less than £5.