if u don't have to pay for it yourself, and your parents dont mind, go with plywood.
If you have to pay for it yourself, get canvas, 8" carpenter screw things, 100ft of rope (NOT NYLON), and a grommet maker.
-Cut the canvas into whatever size you want it to be.
-Make grommets in all corners.
-go to the place u want to set up the bunker.
-hammer the screw things into the ground (through the lower grommet holes)
-Tie the rope around something (through the upper grommet holes)
My field