That would be amazing!! Actually a v good idea as you said, it has all those nooks and crannies for bacteria too. I'm very happy to bash things up a bit to get them perfect. Will PM youLava rock is probably the best material you could use to create raised banks as it'll end up housing a lot of bacteria within it too in the long run.
I have some spare if you like. You may need to smash up the rocks so they're smaller and can be put into filter bags which can be more malleable if that makes life easier for you, but I've got some sat around doing nothing.
What I have might not be enough for what you want, but I have a few kgs of the stuff you could put to use.
Hammer and chisel will easily break up the rocks.