I know what you mean, my first one is heavy and not porous at all. But I beg to differ. The weight and porous of the rock depends on how it is made (Rock salt added to the sand creates holes as it dissolve to make it porous for bacteria, centre can be made hollow, plastic shaving can be added to reduce the weight.
But true you can buy them (at the price of your pocket and the reef). But our reef is a limited resource. Wouldnt it be better if I can help by using less rock from nature and make some on my own plus have fun doing it, save some money and save some reef.
Plus if this works, can spend the money somewhere else! he he... Provided it is successful. Anyway I like to diy, like a hobby to me. More satisfaction when it works. But like I say, just an experiment to see if its feasible. Cos intending on 6ft. Thought of hunting 200kg of rocks that I like sounds daunting. Most of the rocks in the LFS are not very porous and heavy anyway, you have to choose those lighter and porous ones and it takes time and effort visiting several LFS making severval trips.
If I can make them, I can make them in the shape I want. They can also be made to interlock with one another for better integrity. But this is not for the impatient, as it would take months (to cure and for live to grow).