Idk about you but i usally cant just grab a coconut. good idea if i could find one within 400miles. But any way if you have any flat rocks you can build them up or use aquarium glue on some dry rocks to hold them together then put them in.
I was reading it an think man this guy must live in the middle of nowhere, if you live in detroit, you can get a coconut. you dont have to pick one yourself
just go to the garden section of Walmart and buy some terracota clay pots, take a hammer to em to create openings. Lay them upside down or on their sides to create caves. put rocks and plants around them to create a more natural look or even pour gravel on top to create a buried cave look