Here is a card model I did a while back, based on the Warhammer 40K terrain kits that got released a while ago.
Print out on standard cardstock, fold over on the black line and glue together. Then fold on each of the main vertical lines for a piece of free-standing defensive terrain. You can either cut out the firing slit or if you're lazy you can just fill it in black.
We need dakka... LOTS OF DAKKA!