What I did for my DIY Coconut cave was:
1. I nailed three holes in the top to drain the juice/milk.
2. Next I sawed it in half.
3. I let it sit out in the sun so that the "meat" would dry up.
4. Chipped the "meat" out. (Very easy when hard.)
5. Boiled the coconuts to get the tannis out.
6. (Opitional) You can use a saw to saw a small doorway in it, but I didn't do that.
7. (Opitional) You can also layer javamoss on the top and put a tight fit bowl around it, then put it in a dark room for about 3 weeks and you will have a nice dense layer of javamoss solid across the top, gives it a cool effect. (Only optional if you have a planted aquarium.)