Hey Mattomlin
I cant remember how the bunkers and trenches were from that scene, but making trenches are quite easy. Firstly get your base ........ a photo frame is ideal. Then build your trench reinforcements. These are usually made of timber and supporting posts. You can use ice lollipop sticks, stirring sticks or balsa from the hobby store. Its a good idea to design these on paper first, so you know what you are doing (measurements etc). Depending on what height you want, have about 4or five flat then two support posts. For these use the same sticks, thicker balsa posts or even small straight branches to look like posts cut from nearby trees. It all depends on your reference and what you want. Even diagonal support boards can be used. Dont make all the sections the same. Then make more of these sections and make you total trench walls, on both sides. When the trenches you want are ready, make a frame around the base the same height ........ a heightened frame looks much nicer than seeing all your packing and side views of your ground work. With the frame and trench complete, now fill up the areas in between with expanded polystyrene (from packaging) or those ceiling tiles available from hardware stores. Now use spackel or polyfilla to cover over. Build some lumps up in front of the trenches where the soil was dug out as well ..... extra cover! Now paint. For tin ...... use corrugated cardboard, this usually can be bought in hobby or art stores. If the bunkers have a certain style of slab covering the nests .... build these likey did in real life ...... cast them from hardwall and place, then cover with your ground cover as wanted.
This is very generic, but should help somewhat. With your reference, you can design it as wanted, but dont be afraid to use a little artictic license as well to suit, practically or for better appearance, suit of figures or artillery.
A good site for explaining all these with pictures is
Docīs dios. I find it very useful and most of these tips are explained better there. Feel free to ask more questions if this is unclear! Good luck!!