Sand Bag Trench Bunker
With trenches you want to keep in mind that they’ll probably need to be at least 4 feet deep and 3 feet wide. Trenches make things a lot more interesting but due to the amount of digging probably wouldn’t be too practical to use for ALL of your bunkers. I would suggest using a mix of all the different kinds. The trenches would work well as forward or main bunkers because of the low profile and low exposure to offensive forces.
The plywood boards just add a sort of realism that makes it a bit cooler. It also helps to keep things safer so you can run across the trenches without having to worry about falling in. I would think about having some ground level planks running across the top of the trenches in a few areas where you can run across. If you’re going to do this then the planks would have to be flush with the ground and the trench would have to be gradually dug out deeper in the spot where there’s a “low roof” so you’re not running along and WHACK! catch a board in the forehead.