I made some of the bowling ball fish habitat a few years ago. My son in law worked at a bowling alley and the cracked and broken balls he gave me to try. Well they looked good nothing wrong with them. They were a little harder to make than the wood ones I made the other day and my son in law doesn't work at the bowling alley anymore. And I also had to use bricks to sink them because some of them did float. Wood is a whole lot easier to use and a person would not have to buy 4"x6"x6" wood either you could use a cut off of a small tree and just drill random holes in it and stick PVC pipe into them or cut limbs and stick them into the holes.
In case anyone would like to try and make the bowling ball type of fish cover here is the DIY site:DIY "Sputnik" Fish Attractors from Bowling Balls & PVC | Creating habitat | Pond Boss Forum