Try building a pillbox
It seems that you can use many building material types to construct it: block, brick, bags of cement. Excavate and construct and rebury to almost completely hide it, or leave half exposed, or just build on level ground.
Someone with rudimentary skills should be able to pour concrete and rebar to reinforce. Make it as big or small as you want. Stock it with food and live in it or use it as an emergency shelter for short term trouble. Run water and electricity to it underground. Your choice.