That, Cobalt, is a fantastic idea!! With what I think I've decided on for the other side (how to fill the large void and what to fill it with) I'm thinking maybe a shallow relief carving of an osprey's head. The hooked nose of the stick will accommodate the osprey's bill perfectly, I believe, and the stick head thickens just behind where the bill would end giving it more depth.
For the other side, I think I'm going to try a small carving of a fish suspended in the clear resin, maybe with a bit of coloring to make it look like it is an underwater photo. I'll probably paint the back wall very pale blue/green and leave the gnarly bark growth as is to simulate rocks. Maybe chuck in a few slivers of green paper or pine needles to simulate pond grass.
I picked up some clear casting resin and casting putty to make a form for it. Now I just need warm weather.