Bunkers can be made out of anything that will stop a paintball. So with that in mind there are a ton of things you can use to build a bunker with. Here are a few examples of some practical bunkers made out of things you can more than likely get for free somewhere:
In this diagram the hinges are placed on two edges of the triangle to allow it to fold up flat. It’s just as easily made with pallets. All you need are some screws and basic hinges. These bunkers can be made for about 20 dollars each depending on what materials you use.
I also illustrated how you can make use of old used tires. If you can find a good amount of them you can stack them up. Also you may want to consider standing some up on the top row. Old tractor tires(the huge ones) could also be just as easily used if you can find them(and move them).
Here’s some more misc object bunkers:
In this one I showed how you can utilize old carpet and some old 50 gallon barrels.