If your still set on rubble rock, Seachem makes biomatrix media, which is basically rubble rock. I ran mine with a 1 litre bottle in a large media bag for months with no issues. And with it being in a media bag you can pull it out, rinse it off, and suck out the detritus that settles in the chamber.
Seachem Matrix media @ PetMountain
The 1 litre size is all you need, and you can still run a media rack. The only reason I still have mine in the tank is I use it in my QT filter.
There's so much with these that can be done and trust me, I've spent hours and hours reading on things to do. I did remove the sponge between chamber 2 and 3 also. Honestly the last thing I'm going to do is change the return pump to be able to split the return but other than that I'm happy.Thanks acexotix, good to know. I'm trying keep keep it simple as well. I believe I might just be over thinking it. Lol.