Still a favorite of mine are 55 gallon size plastic barrels, the kind used for liquid agricultural products. easy to build permanent and non permanent bunkers with, easy to move around, but they take up lots of space in a truckbed so they take a while to get from farm to the site. my go-to for permanent wood bunkers is spool ends, they are good thick laminated wood which makes em easy to stick to trees, and stick plywood etc on top of (gotta cover up the center holes at least). I would love to find a supplier for the plastic pallets, would be ideal for so much stuff.
Also, wrecked vehicles are fun. they make up some of my favorite bunkers at MRP's City and Carnage fields, and they are easy to get into place if they are still running