Well I realised I had forgotten all about the first bunker I ever built, the good ol' log bunker. All it takes is a hatchet(or a kabar as I used to make mine) and some elbow grease.
For this bunker, the design is simple. For posts/stakes and then a series of logs stacked up in between them to form a wall. Where do the branches and brush come in? Well that's what you use to fill in any significant gaps.
To start just find yourself some over grown christmas trees and hack em down. The stakes/poles ares going to need to be about 6 feet tall, so make sure to cut them at about 6 and a half feet to leave room at the end for sharpening. Sharpening the end of the stakes makes it easier to pound it into the ground. Another note, a good size pole is gonna be about 3-4 inches in diameter.
When you pound the stakes/poles into the ground make sure they down "Y" out but sort of slant towards eachother(even crossing eachother towards the top works well to keep the logs in place).
Then all you have to do is find some good logs(maybe about 5-6 inches in diameter - too big and it'll make it a bit trickier) and stack em up in between the posts.
Here's a couple of pictures of the first bunker I built to give you a better idea:
I apologize for the poor picture quality.
Most of the time you can lash with rope, or fasten with screws and metal straps, but sometimes you also need to weld (50gal drums)
Just my 0.02
That's a good point. Gotta make sure everything is safe and secure.