In this diagram I just tried to give you a few examples of how to use trees to make simple but effective bunkers. You can dig them out or not depending on what your preference is:
The easiest way to make a bunker is just use some 3/4″ or even 1/2″ plywood and nail it to trees. In the diagram I say 3′ by 3′ sections of plywood but the dimensions can be whatever you want. Since these bunkers weren’t designed with camping in mind they don’t have any “windows” or openings to see out of. They are just small enough to where you can snap out of the sides. Some could be designed as “Standing” bunkers with walls high enough to where you can stand behind them or others could be dug out for ducking into on the run.
In one of the examples there’s a set of semi-staggered trees with boards nailed at differen’t angles. This is a nifty setup that I had implemented on our field that works rather well for a forward bunker if you’re looking for an advancement bunker and you know where the enemy is. The angles of the boards would need to be adjusted after you play a few times though to get the feel for what works best.
With the dug out tree bunker, I’ve found that if you stack up layers of log rounds(1′ sections of logs) it works really well to keep the dirt walls in place. You can put down a layer of log rounds then a layer of dirt, then another layer of log rounds etc.