We had to get real creative to make our script work; we had a huge battle at the end of the script that was supposed to take place in trenches and natural dugouts. Well that didn't happen so we went with these slightly large tank traps. Below is a still from the movie. This isn't much of a tutorial other than it might give you ideas.
long-shot
Sorry for the crappy pics, they are screen grabs. Below is two wooden frames stacked on each other, we ended up assembling the other 5 on set. You can see our future bunker (rubber maids) in the background.
frame
This is the foam that we used to cover the outside. It came folded up in a large bundle and cost about 45 bucks, we need two for the project. It is some kind of insulation from Home Depot. It was much cheaper than plywood (and flimsy!).
pinkfoam
Here is the nice PINK tank trap all duck taped together. We ended up having lots of duct tape failure after a full day of pouring rain. I think we would have been fine otherwise.
taped
I still haven't calculated how much i spent on paint but it was around three to four hundred dollars. It just took so much more than i expected, so much all of our fingers were sore from the spray cans the following day.

painted
We had figured out the tank traps but we still had no idea what to do for the bunker. I had a ton of these rubber maid buckets for uniforms and we just didn't have any other better ideas so our bunker is Rubber Maids and lots of paint.
bunker
And here we are. We also painted these foam sheet to look like metal plates, those were really cheap (Home Depot again). We glued two together to make them somewhat stiffer.
bunker-face