Wood bunkers are super easy to build with chainsaws, fallen logs / dead trees and twine.
Find a set of trees that are near each other, ideally if you can find two close enough to each other that a log will fit through then that will help things out.
Fill in the gaps with the deadwood or chopped down dead trees and use the chainsaw to trim it. Tie it all up with good strudy twine.
We've done this lots of time out here in the West and after some practice you can put up a good sized bunker in less then an hour.
Foxholes and trenches we've found tend to be unusable after a season or even a month or two depending on the weather. Holes in the ground tend to get filled with water easily. We've built a few MG bunkers on the sides of hills and ensuring that there is some drainage. For those dig out a wedge of earth from the hillside and cover the top in branches/deadfall. Again tied up with twine.
Those are some easy ways to enhance a field with local items.
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