It depends on how fancy you wanna go, how the fortifications in the end should look and be like in game and what location you're at.
I've seen palisades built with pallets, simple wood blankets layed out on the ground or a rope/string from pole to pole. If you want to build something like a concrete wall, you could try to get a gray foil and lay it out on a wooden frame. Instead of foil you can also use cardboard, as it is easier to paint on, but would most likely suffer during rain.
A combination of these has been e.g. be used on Mythodea 2011 at the Water Camp. The front gate was a full built up structure with wooden palisades at the side which faded to palettes, wooden frames and to strings from pole to pole (and out-time gates with only wooden blankets at the flow, so that you could easily pass through).
If you have the possibility to go full-blown, with e.g. scaffolding, that's a possibility, too.
Tents are also the multi-purpose weapon for displaying structures and buildings. They are cheap, easy to transport, setup and could even be colored or painted. If you have the possibility to get real structures, take it. Abandoned factories, industrial or living areas make a great setup, too. At least if you want the players to dive into a destroyed world.
For computers and similar I'd use wooden/cardboard boxes as these can be easily built, painted upon, transported and setup. Most of the time a GM needs to be available when using these devices anyway or the players will use them on their own, with calling for GMs as necessary. Additional details like a Plasma Ball and some LEDs can make it look pretty cool.
In your case I'd especially go with tents, maybe let the players even bring their own stuff, as it will be their camp. If you can get your hands on metal boxes/crates, those would make good supply containers.
