I don't have a suspended cave per se in my tank, but I did make a small fry refuge out of blue foam insulation (not styrofoam) and plastic screen (the kind to fix a hole in a screen). It's not the most attractive thing, put the small fish like to hide in there and are nearer to the light (and warmth). Over time, some of my Java Moss has attached itself to my structure, along with a little algae. I even have a couple small pieces of Hornwort growing in there.
Maybe you could use similar materials and attach some plants to it to hide the plastic screen? My structure is loosely attached to the side of the tank with a twist tie. It floats (mostly in one place) and seems to make the fish very happy.
I can try to post some pictures if people are interested. Also, that plastic canvas some of us used to make crafts as kids works well in tanks too.