Something I've seen done in actual fish tanks, but will work just as well here:
Get a terra cotta pot. Either tip it on its side, or turn it top-down and use pliers to chip away at the lip to have a small cave opening (make sure you take off small s[ieces at a time, or the entire pot will crack). Take silicone (boat or aquarium sealant is fine) and spread a layer on the outside of the pot and attach the rocks. I have done something similar, but instead of rocks, I used sand to cover the pot. I use it in my aquarium (I have a pic if you need a visual).
I hope this helps a little.