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How about a sheet of clear plexiglass right over the blue bottom?

It would give you the illusion of depth to the ice and you could frost the edges to make it look like snow has drifted out onto the ice.

You could probably paint some real light lines (cracks) on the underside of the plexiglass too.

And for the cavern you could put the plexiglass over the top or right under the top layer. That way you could see down into it like it was a real ice cavern.