My clowns also host in a clam... I wish I had a picture of it... it's kind of cute. They just kind of lay on the mantle and the clam doesn't seem to mind that much, which is surprising to me. If the clowns get too rough the clam will "shrug" and the clowns will swim away for a bit and then come back.
What is kind of ironic is that there are lots of other things that you would think would make better homes... like a very large crop of branching frogspawn, or a large toadstool leather that is about the size of a fist... but no... they like the clam... strange.