Ordinary fired ceramic tile. Even clay gardening pots have worked.
Where is he cleaning? I have removed rocks just before the hatch and had hundreds of surviving larvae in my rearing tanks.
We have also successfully removed larvae after the hatch, when removing the rock was impractical.
Want to know more?
I'm not an expert. Others have far surpassed me, but here's a pic of the start of my first success, back in 1996.