Substrate to grow more pods in refugium

[trilinearmipmap]

I'm looking for something to put in my refugium to provide surface area to propagate pods.

The refugium is growing Chaetomorpha which floats on the top. On the bottom is an inch of sand.

I'm thinking along the lines of some kind of porous rock type stuff, I would use live rock but $$$ and not available locally. Maybe "Tufa Rock" or something similar, does anyone have any ideas?