my favorite bunkers are dug out about 2 feet with the dirt piled up to about 1 foot in front of the hole. Then you can use 2x6, 2x8, 2x10, or 2x12 planks laid on top of that to make the bunker taller.
Carpet stretched between 2 trees has worked well for me too.
Pretty much just scrounge stuff up from dumpsters, garage sales, etc. I used a headboard and footboard from a bed and they worked great.
That's just about all the creative stuff I can think of.
Here's an example of one of the bunkers that I described first. It's dug out about 2 feet deep with about 2 feet of boards stacked up. Each side of the bunker is about 8-10 feet long so this thing can accomodate 2 people. Bunkers like these are tough to build but they really add to any field and I love to play on them.
and a decent load of wood from the dumpster of a small house being built. I took about 3-4 loads of wood from that dumpster that were all this large. Provided enough wood to build plenty of nice bunkers.